Non-bulk preloading files: faster Elvis

[fenollp]

Could we

1. load file & apply rules (then repeat with next file) per-file thus breaking the current "load all files & then apply rules"
2. make sure to not load same files twice for a same set of rules (dirs+filter can find the same files more than once)

For projects with lots of files using Elvis has proved to take quite a lot of CI time.

Using Elvis 0.2.11.

[fenollp]

Actually sometimes (one in five?) Elvis will hang during CI. Check out https://circleci.com/gh/2600hz/kazoo for some examples/code