Stop printing to stdout/stderr, but return elements we can work on top of (for `elvis` and `rebar3_lint`)

[Licenser]

elvis:rock/* seem to include all the printing code as well as the checking code. It could be helpful to separate this out in two step that way the output form the 1st step could be programmatically consumed while the output from step 2 could printed to the console.

[paulo-ferraz-oliveira]

From what I gather from https://github.com/inaka/elvis_core/issues/4, elvis_core should not even print, and let that for e.g. rebar3_lint, so maybe when we pick this up the goal can be to remove printing while adding it next to rebar3_lint (and elvis).

[elbrujohalcon]

I agree