Support two kinds of history: cmd and search

Rename history into cmd.

Add search history (which runs the search on execute).

Required lots of minor changes because I moved the files into a common history/
folder. Not sure how to share code between them. Seems like most
code is configuration.
Rename CtrlPHistory -> CtrlPCmdHistory

Makes the command unambiguous, but breaks backwards compatibility.
Merge pull request #1 from pydave/master

Support search history