confit: replace with a wrapper around confuse

This re-exports all of the confuse module under `beets.util.confit` and
patches in the minimum subset of commits to confit that have not yet
been ported to confuse such that the test suite passes. A warning is
issued when importing from the deprecated namespace.

setup.py

@@ -91,6 +91,7 @@ setup(
         'musicbrainzngs>=0.4',
         'pyyaml',
         'mediafile>=0.1.0',
+        'confuse>=0.5.0',
     ] + [
         # Avoid a version of munkres incompatible with Python 3.
         'munkres~=1.0.0' if sys.version_info < (3, 5, 0) else

tox.ini

@@ -25,6 +25,10 @@ deps =
     coverage
     discogs-client
 
+[flake8]
+per-file-ignores =
+    beets/util/confit.py:F821
+
 [_flake8]
 deps =
     flake8