Rule for message filename is too strict

[LukeShu]

In api.py the regex MDMSG_FILENAME looks like it tries to extract information from the "unique name" section of the filename, resulting in it potentially discarding valid files (such as those created by offlineimap).

Further, to quote the Maildir spec:

Unless you're writing messages to a maildir, the format of a unique name is none of your business. A unique name can be anything that doesn't contain a colon (or slash) and doesn't start with a dot. Do not try to extract information from unique names.

To comply with this I've written the following regex; I figured a 1-line change wasn't worth a pull request.

MDMSG_FILENAME_PATTERN = "(?P<key>[^.:/][^:]*)(:2,(?P<flags>[PRSTDF]*))?"

[nicferrier]

I'd be really happy for you to fix this... I am no longer supporting this code. See the README.creole for details.