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

lib/pinchflat/utils/function_utils.ex

@@ -0,0 +1,15 @@
+defmodule Pinchflat.Utils.FunctionUtils do
+  @moduledoc """
+  Utility functions for working with functions
+  """
+
+  @doc """
+  Wraps the provided term in an :ok tuple. Useful for fulfilling a contract, but
+  other usage should be assessed to see if it's the right fit.
+
+  Returns {:ok, term}
+  """
+  def wrap_ok(value) do
+    {:ok, value}
+  end
+end