Generic <impl>_MODE setting. Add travis modes.

Generic <impl>_MODE setting. Add travis modes.

For implementations that support different compile/run modes, the
convention is not <impl_name>_MODE=<mode>. This is passed through to
make/docker commands. Fix Makefiles and run scripts to listen to the
respective *_MODE variables.

Enable travis builds/tests for 4 Haxe modes and 2 python modes.