Implement convenience commands for actions #67

Right now you can do something like:

actions:
  clean: "rm -f *.class"

But that obviously won't work on Windows.

So we can implement a whole system for executing different commands on different systems (see #66) but for convenience we could add a couple of commands that everybody is expected to use. For example:

• {rm} - if the first part of a command is {rm} it will be translated to rd on WIndows and stay rm on Linux/Mac. It takes a list of paths, but it won't support any options. It just implements rm -rf and rd /s /q, period.
• {cp} - is probably another good one. No arguments either. Probably need to do something clever with target being file or folder.\
• Any others?