make default --save semver option configurable

Implementing suggestion in npm/npm#4713

You can return old npm behaviour in config:
yapm config set save-range '~'
yapm install --save express

Or just use range shortcut in the command line:
yapm install --save --range='~' express

Available options for save-range are:
 - "~"
 - "^"
 - ">="
 - "<="
 - "=" - no range (exact)

lib/install.js

@@ -350,10 +350,13 @@ function save (where, installed, tree, pretty, hasArguments, cb) {
         if (u && u.protocol) w[1] = t.from
         return w
       }).reduce(function (set, k) {
+        var range = npm.config.get('save-exact') || npm.config.get('save-range')
+        if (typeof(range) === 'boolean' || range === '=') range = ''
+        else if (range === '>' || range === '<') range = range + '='
+
         var rangeDescriptor = semver.valid(k[1], true) &&
-                              semver.gte(k[1], "0.1.0", true) &&
-                              !npm.config.get("save-exact")
-                            ? "^" : ""
+                              semver.gte(k[1], "0.1.0", true)
+                            ? range : ""
         set[k[0]] = rangeDescriptor + k[1]
         return set
       }, {})