Use `#shellwords` when parsing TM_SELECTED_FILES.

Fixes #87.

This seems the correct way of dealing with it, see e.g. https://github.com/textmate/textmate/blob/2eaa97b1dc3a1df58a172769bc9646fb4a0610d2/Applications/TextMate/support/Bundles/Avian.tmbundle/Commands/Show%20Images.tmCommand#L14