Added support for JS-Only loader (fragment)

Hi, thanks for this nice library and here is my proposal for an enhancement: 

**Reasons for the Update**:
 - I needed to generate a JS file instead of the HTML loader fragment file.
 - I needed the generated file to be placed in a custom directory rather than the default.
 - I needed the javascript source code to be raw (not uglified)
 - I needed all of these to be configurable so I can default back to generating HTML fragments when necessary.

I've altered the `Iconizr.prototype._renderLoaderFragment` method, so that it enables the following changes/features without disabling any existing ones or default behaviours. So, if the following configurations/options are not set at all, **iconizr** will just behave the expected way.

**Changes**:
 - Added `options.loader`
   - `options.loader.type` (String) Valid values: `"html"` or `"js"`. Defaut: `"html"`  
  Specifies the type of loader file to generate. Set to `.html` if `<noscript>` fallback is required.
   - `options.loader.minify` (Boolean) Defaut: `true`  
  Specifies whether to minify the JS loader code.
   - `options.loader.dest` (String) Defaut: `""` (empty)  
  Indicates the directory for the generated loader file (relative to the output directory).

I couldn’t see any contributing guidelines but, still tried to keep up with your coding style (tabs, indents, etc)... Also tested with `grunt-iconizr`.

Note: I might optimize this further once accepted.

Best,
Onur Y.