R1:e00fae8436b5

Files can be set not to use 'ignore-all' by default.

Summary:
Python people don't seem to like the 'ignore-all' as default. Provide a way
to configure which file types should not use 'ignore-all'.

Test Plan:
Tested that it worked with bunch of Python of files and non-python
files. Cache was disabled during the test.

Reviewed By: jungejason
Reviewers: epriestley, jungejason
Commenters: epriestley
CC: aran, jungejason, epriestley
Differential Revision: 713