Request: please consider tagging versions

[SamWhited]

Hello, please consider tagging released versions on this repo. Currently it is hard to find what version of the source in this repo corresponds to what version on NPM when building source packages for various Linux distros. Tagging this repo would allow us to easily pin to a specific tag instead of having to try and figure out what commit to pin to. On a related note, NPM doesn't provide an RSS or Atom feed for versions, while GitHub does for its releases making it much easier to discover updates and bump packages.

Thanks for your time!

[Bouke]

Especially when things break, now with 1.5.1 to 1.6.0 where cli arguments seemingly have no effect on how clean-css is invoked.