Fix isComposite with empty glyphs

Was broken in 626107c8.
Add support for dumping XMLs to the standard output.
[subset] Fix order of chain for consistency

I'm confident I haven't broken anything.
[subset] Handle case of None ChainContext class with non-zero chain

It would be a font bug, but handle regardless.
[subset] Handle None ChainContext

Fixes https://github.com/behdad/fonttools/issues/134
[merge] Support Extension lookups
[merge] Add preliminary support for BASE table