Broke main.go out into a subfolder for easier build instructions

Hi there! Someone suggested that I build a `cmd` subfolder for my own Go utility that is also usable as a library.  I have been happy doing so with mine and I thought I'd propose the same for your utility as a way of streamlining the `go build` process.  This might break your drone.io config though - I don't know that I can update that via PR.

Thanks for the neat tool - I'm happy to abandon `ag` for a Go version if only because I need more Go code to read.

:beers: