help command linux support and extra searches

* Linux support. Opens default browser. Default to and does not change behavior of Mac OS X.
* Extra searches with type:query syntax. Type is optional. Defaults to the original MDC search.

Extra searches are:
* safari - Apple Safari developer center
* opera - Dev.Opera
* w3c - Google search limited to w3c specs
* ms - MSDN search limited to Web & IE
* caniuse - compatibility table at caniuse
* quirksmode - quirksmode compatibility tables

Anything else will default to MDC.

The intent is not to turn stylus into a general search tool, but to include help command support for the most important core CSS references.

Thanks for stylus!

Cheers