Added a tuple type

Support was already pretty much built-in, but was not surfaced.

src/index.js

@@ -1,5 +1,6 @@
 //Export types
 export { default as union } from './types/union';
+export { default as tuple } from './types/tuple';
 export { default as Any } from './types/any';
 export { default as Nil } from './types/nil';
 export { default as reference } from './types/reference';

src/parse/parse-object.js

@@ -21,7 +21,7 @@ export default function parseObjectType(options, type, arrayType) {
     , virtuals    = {}
     , methods     = {}
     , meta        = {}
-    , kind        = arrayType ? 'array' : 'object'
+    , kind        = arrayType ? ( type.length === 1 ? 'array' : 'tuple' ) : 'object'
     , prototype
     , thisType
     , restType
@@ -290,6 +290,8 @@ export default function parseObjectType(options, type, arrayType) {
         , packed
         ;
 
+      name = String(name);
+
       if (propNames.indexOf(name) !== -1) {
         type = properties[name];
       } else {

src/types/tuple.js

@@ -0,0 +1,22 @@
+import parseObjectType from '../parse/parse-object';
+import { isArray } from '../utils';
+
+export default function tuple(...types) {
+  if (!types.length) throw new TypeError('Tuple requires item types.');
+
+  if (types.length === 1) {
+    if (!types[0] || !isArray(types[0]) || types[0].length<2) throw new TypeError('Tuple requires multiple item types.');
+    types = types[0];
+  }
+
+  function Tuple(options) {
+    let type = parseObjectType(options, types, true);
+
+    type.name = `tuple(${Object.keys(type.properties).map(prop => type.properties[prop].kind).join(', ')})`;
+    return type;
+  }
+
+  Tuple.isType = true;
+  return Tuple;
+}
+