Updates & PostCSS integration (up-to-date Autoprefixer)

[S1SYPHOS]

Hey there, I really like your this starter project and want to contribute, so here goes:

• Replacing gulp-autoprefixer (since it's deprecated) with autoprefixer (now part of the PostCSS ecosystem)
• This also allows for easy integration of other PostCSS plugins people might find useful -- see http://postcss.parts/ (totally optional)
• Adding PostCSS as main feature of your starter project

Additionally JS linting might be nice to have, I prefer ESLint over JSHint, just let me know! Good day to you, Sir

S1SYPHOS

[ixkaito]

@S1SYPHOS

Thanks! I'll merge this after I fix bugs of the test.

I also prefer ESLint. Looking forward to your PR :)