Allow pw.token to be stored outside config file

[jrn]

I would like to be able to configure pw.token to name a file or program producing a token. That way, if while debugging I send someone output of "git config -l", then I won't expose my token.

For example, if git-pw could obtain the token using a credential helper http://www.kernel.org/pub/software/scm/git/docs/gitcredentials.html then that would be perfect for me. (As a bonus, that would allow other people to store the token in their platform's standard keychain.)