Use WebP binaries directly

Don't merge until https://github.com/1000ch/node-cwebp-bin is moved to imagemin org and I've pushed some fixes for that.

Fixes #2.
Theoretically, it maybe could. But I'd have to rewrite it. Currently it takes the buffer, writes a temp file and executes the process using execFile, https://github.com/kevva/exec-buffer/blob/master/index.js#L82-L98.