Introduce CustomConfig method that applies set of transforms

It is a bit cleaner than making chains of calls like
{{{
value = os.path.expanduser(value)
value = os.path.abspath(value)
}}}
since we do see all transformations to be applied in a single
iterable and have no repeated code like in the above example.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>
Enable tilde and environment variable expansion on some items

Expand environment variables in the following configuration items:
- general.pythonfile;
- general.metadata;
- mbnames.filename;
- Repository.localfolders.
- Repository.sslcacertfile.

Make tilde and environment variable expansion in the following
configuration items:
- Repository.sslclientcert;
- Repository.sslclientkey.

GitHub pull request: https://github.com/OfflineIMAP/offlineimap/pull/113
Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>