Bug 1375166 - [mozlint] Don't require leading '.' in extensions, r=standard8

This was a regression from bug 1288432. The 'extensions' config in mozlint required a
leading period, but eslint requires them without the period (and this got copied over
to the linter definition). The result was mozlint filtering out any files (not dirs)
that were passed in.

This just modifies mozlint to strip out the period so both are acceptable.

MozReview-Commit-ID: CbNynYzrbGz

python/mozlint/test/test_parser.py

@@ -35,6 +35,8 @@ def test_parse_valid_linter(parse):
     assert 'description' in lintobj
     assert 'type' in lintobj
     assert 'payload' in lintobj
+    assert 'extensions' in lintobj
+    assert set(lintobj['extensions']) == set(['js', 'jsm'])
 
 
 @pytest.mark.parametrize('linter', [