Use tab for “Open directory in Terminal” (optionally).

If `TM_TERMINAL_USE_TABS` is set, the command now creates a new tab in the frontmost terminal window (instead of opening a new terminal window).
Default to new tab for 'Open Terminal'

Set `TM_TERMINAL_NEW_WINDOW` if you prefer to open a new window instead.
Merge remote-tracking branch 'upstream/master'
Rest “Open directory in Terminal” to upstream.

Upstream now defaults to open a new tab, so no need for a customized version anymore \o/