Import variant commands from various versioned sources

[mumoshu]

So that you can create a common/library variant commands to be versioned and reused.

tasks:
  anothercmd:
    # Basically runs `go-getter github.com/foo/bar?ref=v0.1.0//dir /tmp/somewhere` and then include the command definition from /tmp/somewhere/anothercmd
    import: github.com/foo/bar?ref=v0.1.0//dir/anothercmd

This can be an implementation detail, or the first step of #5

[osterman]

Yes!! Want this badly.