Add a .reviewboardrc option for --svn-prompt-password
Add a .reviewboardrc option for --svn-prompt-password

`.reviewboardrc` now accepts the `SVN_PROMPT_PASSWORD` configuration key,
which maps to the `--svn-prompt-password` commandline option.

Testing Done:
Set up a SVN repository with `SVN_PROMPT_PASSWORD=True` in the
`.reviewboardrc`. I was prompted for my password when running `rbt
post`.

Bugs closed: 4446

Reviewed at https://reviews.reviewboard.org/r/8336/

reviewboard/rbtools,reviewboard/rbtools,reviewboard/rbtools