Adds ability to pass CLI args for configuration

[tylergaw]

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

[tylergaw]

@cognitom Also, I really love this project. I've used it a handful of times and it's excellent. Thank you!

[cognitom]

@tylergaw Thank you for your pr :-)
I think we need some discussion about it. This repository has started as a template, not as a plugin. And I preferred to keep it simple. I have two questions. Any comments are welcome.

• Are command line arguments familiar for "designers"?
• Should be a template or a plugin?

[tylergaw]

@cognitom Cool, thanks.

• I think you're right, I don't think CLI args are the most familiar thing for a lot of designers. With this there may be more exceptions though because of the non-trivial install process. If a designer has enough knowledge to get everything installed and can call gulp maybe they're comfortable with the args?
• Not completely sure. I added the CLI args to my fork for two reasons: 1) I found myself using the gulp file for multiple projects and I didn't want to have to keep manually editing the file. 2) I've started tinkering with a GUI for this where you can set all the options using standard OS X controls. For that to work I needed to be able to pass the options as args.

I could see it going either way. The CLI args may add too much complexity to an otherwise simple template. Or it could make it easier for a small group of folks to use.