add flag to recurse into subdirectories

Was wondering why this did not work. Fixes #58
I don't think that we are shooting for that:
```
# go list golint/golint.go 
command-line-arguments
```
And I seriously do not want to have 1:1 compatibility with the go list command since it just supports import paths and relative import paths. I think the current feature set of this PR is just fine. Everything besides "..." between directories works and I am willing to implement that too and send it to the Go project since there is already a comment that they wanted that in the first place.