mime: No trailing comma in a URL
The regex that Balsa uses to identify a URL in a plain text message part already omits a trailing period ('.') from the match, as it is usually punctuation and not part of the URL. This commit additionally removes a comma (',') from the last character match, for the same reason: more likely to be punctuation. This of course breaks the identification of a URL that *does* end in a comma, but that appears to be less likely than erroneously including one. Discussion at #59.