Add languages option feature

As we discussed here: https://github.com/Workiva/wGulp/pull/63#discussion_r19161225

Creating the PR into the depTree branch so this portion of the code can be reviewed and discussed separately.

This allows us to specify a languages option in a wGulp project like so:
```js
var customizedOptions = {
    languages: ['javascript', 'typescript']
};
```

The defaults are to include all supported languages:
```js
    languages: ['javascript', 'typescript', 'coffeescript', 'livescript']
```

As languages are removed from this list, wGulp will automatically trim the tasks it runs for builds.

@trentgrover-wf 
@evanweible-wf 
@shanesizer-wf 
@theisensanders-wf 
might as well generate this array from the keys in langMap