Adds ability to pass CLI args for configuration

In the current iteration, to make config changes you need to manually edit the `gulp.js` file. This pr makes use of https://github.com/chevex/yargs to allow the available config options to be passed via command line arguments.

For example. To use a different sketch file, custom name, class and distribution location you can:

```
gulp symbols -d ~/Documents/myfonticon.sketch -n my-fonticon -c icons -l ~/Documents/project/fonts/
```

This also adds a no gulp task `clean-dist` that removes all the files and directories from either the default dist location or one given by -l.

Updated the readme with all of the options and descriptions. Added the same behind `gulp -h`