Feature: support custom output formatting

[kpacha]

Hi, guys!

First of all, thanks for building this amazing tool!

I've been using it for more than a year and I'd like to know your opinion about supporting some kind of custom output formatting like the one offered by go list -f in addition to the two current formats. I know hey already generates CSV, but I think this feature would add tons of potential uses of the project (https://dave.cheney.net/2014/09/14/go-list-your-swiss-army-knife).

Maybe it could be done without changing any method signature, just by adding some code to the printer and extending the 'meaning' of the -o flag.

Cheers

[kpacha]

I think this is related to #29