Allow plugins to have a version property.

Don't throw if a plugin is installed more than once as long as the version number is the same (despite the plugins not being === each other).

documentation/api/use.md

@@ -12,6 +12,7 @@ Unexpected plugins are functions or objects that adhere to the following interfa
 Optional properties:
 
 * __name__: `String` - the name of the plugin.
+* __version__: `String` - the semver version of the plugin (string).
 * __dependencies__: `String array` - a list of dependencies.
 
 Required:

lib/Unexpected.js

@@ -504,12 +504,18 @@ Unexpected.prototype.use = function (plugin) {
     });
 
     if (existingPlugin) {
-        if (existingPlugin === plugin) {
+        if (existingPlugin === plugin || (typeof plugin.version !== 'undefined' && plugin.version === existingPlugin.version)) {
             // No-op
             return this.expect;
         } else {
-            throw new Error("Another instance of the plugin '" + plugin.name + "' is already installed. " +
-                            "Please check your node_modules folder for unmet peerDependencies.");
+            throw new Error("Another instance of the plugin '" + plugin.name + "' " +
+                            "is already installed" +
+                            (typeof existingPlugin.version !== 'undefined' ?
+                                ' (version ' + existingPlugin.version +
+                                (typeof plugin.version !== 'undefined' ?
+                                    ', trying to install ' + plugin.version : '') +
+                                ')' : '') +
+                            ". Please check your node_modules folder for unmet peerDependencies.");
         }
     }
 
@@ -519,6 +525,7 @@ Unexpected.prototype.use = function (plugin) {
         throw new Error('Plugins must be functions or adhere to the following interface\n' +
                         '{\n' +
                         '  name: <an optional plugin name>,\n' +
+                        '  version: <an optional semver version string>,\n' +
                         '  dependencies: <an optional list of dependencies>,\n' +
                         '  installInto: <a function that will update the given expect instance>\n' +
                         '}');

test/unexpected.spec.js

@@ -5276,6 +5276,7 @@ describe('unexpected', function () {
             }, 'to throw', 'Plugins must be functions or adhere to the following interface\n' +
                    '{\n' +
                    '  name: <an optional plugin name>,\n' +
+                   '  version: <an optional semver version string>,\n' +
                    '  dependencies: <an optional list of dependencies>,\n' +
                    '  installInto: <a function that will update the given expect instance>\n' +
                    '}');
@@ -5389,7 +5390,58 @@ describe('unexpected', function () {
             expect(callCount, 'to be', 1);
         });
 
-        it('installing a plugin with the same name as another plugin (but not ===) throws an error', function () {
+        it('installing two different plugins that are identically named and have the same version (but not ===) will only install the first one', function () {
+            var callCount1 = 0;
+            var plugin1 = {
+                name: 'plugin',
+                version: '1.2.3',
+                installInto: function () {
+                    callCount1 += 1;
+                }
+            };
+            var callCount2 = 0;
+            var plugin2 = {
+                name: 'plugin',
+                version: '1.2.3',
+                installInto: function () {
+                    callCount2 += 1;
+                }
+            };
+            expect.use(plugin1).use(plugin2);
+            expect(callCount1, 'to be', 1);
+            expect(callCount2, 'to be', 0);
+        });
+
+        it('should throw an error when installing two different plugins that are identically named and have different versions', function () {
+            expect.use({
+                name: 'plugin',
+                version: '1.2.3',
+                installInto: function () {}
+            });
+            expect(function () {
+                expect.use({
+                    name: 'plugin',
+                    version: '1.5.6',
+                    installInto: function () {}
+                });
+            }, 'to throw', "Another instance of the plugin 'plugin' is already installed (version 1.2.3, trying to install 1.5.6). Please check your node_modules folder for unmet peerDependencies.");
+        });
+
+        it('should throw an error when two identically named plugins where the first one has a version number', function () {
+            expect.use({
+                name: 'plugin',
+                version: '1.2.3',
+                installInto: function () {}
+            });
+            expect(function () {
+                expect.use({
+                    name: 'plugin',
+                    installInto: function () {}
+                });
+            }, 'to throw', "Another instance of the plugin 'plugin' is already installed (version 1.2.3). Please check your node_modules folder for unmet peerDependencies.");
+        });
+
+        it('installing a version-less plugin with the same name as another plugin (but not ===) throws an error', function () {
             expect.use({
                 name: 'test',
                 installInto: function () {}