Redo indexing mechanism (#16)

* Bumped up the line length because I fear no man

* Refactored indexing

Previously, indexing worked by collecting the video IDs of only videos
that matched indexing criteria. This new model instead stores ALL videos
for a given source, but will only _download_ videos that meet that criteria.
This lets us backfill without indexing, makes it easier to add in other
backends, lets us download one-off videos for a source that don't quite
meet criteria, you name it.

* Updated media finders to respect format filters; Added credo file

.iex.exs

@@ -44,7 +44,7 @@ defmodule IexHelpers do
         :channel -> channel_url()
       end
 
-    SourceDetails.get_video_ids(source)
+    SourceDetails.get_media_attributes(source)
   end
 end