Better bash-less Windows support in actions.run_shell: add command_bat, command_ps

Description of the feature request:

For better bash-less Windows support,ctx.actions.run_shell could add command_bat (with the same behavior as genrule's cmd_bat) and command_ps (with the same behavior as genrule's cmd_ps).

This would, in particular, considerably simplify skylib's Windows support, where currently we are forced to write and execute temporary .bat files on Windows (and attempt to deal with the ancient terror of argument quoting in .bat interpreter context - take a look at https://stackoverflow.com/questions/4094699/how-does-the-windows-command-interpreter-cmd-exe-parse-scripts if you don't mind losing some points of sanity).

Pinging potentially interested parties: @brandjon @meteorcloudy @alexeagle