Broke `be comment --xml` out and extended into `be import-xml`.

Broke `be comment --xml` out and extended into `be import-xml`.

It should currently do everything that `be comment --xml` did, but it
still has a way to go before it lives up to it's longhelp string,
mostly figuring out bug/comment merging.

The allowed XML format also changed a bit, becoming a bit more
structured.

cmdutil.bug_from_shortname() renamed to cmdutil.bug_from_id().

New functions cmdutil.parse_id() and cmdutil.bug_comment_from_id().

Additional doctests in libbe.comment.Comment.comment_shortnames() to
show example output if bug_shortname==None.

Brought be-xml-to-mbox and be-mbox-to-xml up to speed on the current
<be-xml>-rooted format.
  * Added <extra-string> handling to their comment handling.
  * Moved extra strings from email bodies to X-Extra-String headers
    (some comment bodies are not text, and we should keep the estr
    location consistent between bugs and comments.)