Reply instead of DM'ing if called by unroll

Thread Tree can currently be triggered either by submitting a tweet link on the Web form, or by replying to a thread on Twitter mentioning `@threadtreeapp unroll`; the second of these will trigger a DM to the mentioning user when the thread is unrolled.

Thread Reader, on which concept Thread Tree is based, replies in-thread instead of sending a DM. It'd be nice if Thread Tree did the same.