make reading passwords customizable

We use a process filter to detect when git or one of its subprocesses
(in many cases ssh) prompts the user for usernames, passwords and
passphrases.  Until now the prompt strings we were able to detect were
hardcoded.

Add new options magit-process-{username,password}-prompts so that users
can easily tell magit to forward the (sometimes quite unique) prompts
their setup spits out, without having to patiently having to wait for us
maintainers to agree, to support them.
magit-process-{username,password}-prompts: pimp regexps
forward git/ssh yes-or-no prompts to the user
Merge branch 'prompt'