Gmail parseable token in email notifications (BB-3142)

When Bitbucket sends out an issue change notification. The email
contains the following footer.

{{{

Responsible:
userdave
This is an issue notification from bitbucket.org. Blah blah blah
}}}

Anyhow, that isn't very easy to filter when using Gmail, notmuch, etc. It's just not something that's easy to specify using tokens.

It would be awesome - if the email messages contained a parseable token, something like this:

{{{

responsible-userdave
This is an issue notification from bitbucket.org. Blah blah blah
}}}

Just as readable by a developer. But, can be filtered by Gmail. It isn't currently possible to filter by this line, because Gmail, Outlook, etc cant match something that contains a newline character.