* Allow setting env vars
Also, bring all runfiles of data deps along for execution - currently
data deps which consist of more than one file aren't copied into the
action, which means that they will fail at runtime when they try to use
their runfiles.
* Update docs
* Added `data` attribute for files needed by the rule at runtime.
* Updated documentation
* Added examples
* Enable runfiles on windows
* Also gather transitive runfiles