Disable error about spaces/tabs

Summary:
The spaces/tabs error gets pretty annoying,
especially when we don't visualize the difference.
This disables that (relaxes the rule).
Should I make a new config for it?
I think it'd be backwards compatible just fine,
since its a relaxer, but one never know.

Test Plan: - Mix tabs and spaces in a program, don't see error

Reviewers: john

Reviewed By: john

Subscribers: #cs

Differential Revision: http://phabricator.khanacademy.org/D12364