Add Flatpak support

[ssokolow]

While I'm not sure it'd be appropriate to have cargo-bundle orchestrate all of what goes into Flatpak's build process, something like this would probably make sense for the same reasons #92 has been proposed.

1. Generate a Flatpak project definition (which is a simple JSON or YAML file) that can be edited to supply stuff beyond the scope of cargo-bundle and sync the keys cargo-bundle does care about, thus allowing a single source of truth for what cargo-bundle knows without taking responsibility for everything Flatpak does.
2. Wrap flatpak-builder to provide a consistent means of triggering the packaging process.