add npm clean script to prevent error on build

[wad3g]

Added clean script to remove all files in the build/* directory.

This fixes issue #7 .

[rsp]

@wad3g Any reason why this is not merged yet?

[wad3g]

@rsp No idea. Looks like the only changes the last few years has been just updating dependencies.

I stopped using this port not too long after my PR so I hadn't even noticed. Wanted to get away from Gulp in favor of plain ol' PostCSS & NPM Scripts.

[rsp]

Wanted to get away from Gulp in favor of plain ol' PostCSS & NPM Scripts.

@wad3g Interesting - have you managed to port node-slate gulp scripts to plain npm scripts or are you using something different for your docs now?

[wad3g]

@rsp I built this for a single project so I haven't used or built it month so my memory's a little fuzzy on exactly where things left off.

I had everything working correctly using just npm scripts except for the last, and largest, hurdle -- syntax highlighting. Most of the scripts I used were just from this small/lean boilerplate I halfway put together a while back called spar here's the link to it's package.json if you'd like to see the scripts.

The docs project was on a time crunch so I had to take a few shortcuts and just use npm along side of Gulp and switching out gulp-sass for gulp-postcss. Intended to finish the npm scripts port in my freetime, but you know how that goes.