Fetch more options from elvis.config + rebar.config

[paulo-ferraz-oliveira]

At the moment, elvis_core depends on application:get_env which is not very usable for the consumer, especially if his/her project is not bound to a release.

The purpose of this task is to, while keeping backward compatibility, fetch further options from elvis.config + rebar.config for usability.

Note: search the code for application:.

[paulo-ferraz-oliveira]

elvis_config:from_rebar seems to be called only from tests.