Run on change file .css

[grantand]

Hi!
Thanks for this package!
It would be great to run autoprefixer on when csss files change. Because I work with .less files that automatic compile to .css and I dont save file .css accordingly autoprefixer doesn`t work.

[sindresorhus]

That's not really the intended use-case of this plugin. For what you want it would be better to use some kind of build system, like webpack.