remotepassfile doesn't expand env variables #746
Description
Playing around with offlineimap I stumbled over an inconsistent behavior for path-expansions in the the configuration.
While it is allowed to have a env variable expansion for localfolders and sslcacertfile it is not the case for remotepassfile.
I was wondering if that is intentional, since the way paths are expanded feels a bit badly documented to me. So, my question is, if that behavior can be aligned, or to make that at least explicit in the docs?
IMHO: It's preferable to have a variable expansion for the remotepassfile since that leads to less confusion and would allow entries like: $XDG_CONFIG_HOME/secrets/offlineimap-password.txt.
Furthermore, I don't see any relevant security concerns which exceed the scope of all the other places where env variables are already expanded. Though, it allows a concise configuration following the XDG specification.
In any case I would happily provide a PR if desired.