New gitignore
When deleting a component from a glyph, _endBaseGlyphObservations assumed the base glyph is present in the font. This does not have to be the case. A component can reference a glyph that doesn't exist.

Note: _beginBaseGlyphObservations also makes the same assumption. I'm not sure why I'm not tripping that error.
Merge pull request #11 from LettError/ufo3

This looks good. Thanks!