Add HistorySearchUp and HistorySearchDown actions which are similar to
HistoryUp and HistoryDown but search for the prev/next history item
whose beginning matches the currently entered text in the infobuffer
(more precisely, the text before cursor).
Also fixed the following issue: if we scrolled to an older history item
and then edit the infobuffer, this older item gets modified.
We should not edit old history entries. So in this case set HistoryNum
to the last (newly added) item and modify the last item.
Add InfoBuf's method AddToHistory function which adds a new item
to the history for the prompt type `ptype`.
This function is not used by micro itself. It is useful for plugins
which add their own items to the history, bypassing the infobar
command line.
