contrib: pick: bind M-p and M-n to prev/next thread

This adds functions to go to the previous or next thread to
pick. Prev-thread behaves similarly to prev-message in show: if you
are on the top line of a thread it will go to the top of the previous
thread, otherwise it will go to the top of the current thread. Next
thread will always go to the top of the next thread (or the end of
buffer). These are bound to "M-p" and "M-n" by default (matching the
bindings in show).