Move from `dirs`+`filter` to `files`

[paulo-ferraz-oliveira]

At the moment, we have dirs, files (not exposed), and filter (that are "concatenated" with dirs) to identify files.

Feature request: get rid of all but files (expose it), since we can express everything with that alone.

Also, it's common practice in other libraries so the concept shouldn't be foreign to users and we can always implement it (initially) in a backward compatible way.

[bormilan]

We want to eliminate dirs and filter from the config, and make a files that will accept something like: src/*.erl, right?
(and of course we should handle the old format too, with a warning message that it's deprecated)