dim: Add extract-tags command

Add a command for extracting various tags (eg. Reviwed-by:) from
emails. You can give the comamnd a rangeish to add the tags from
the same email to multiple already applied patches.

The regexp used to pick up tags is purposefully quite broad. People
tend to typo these things, or add extra whitespace etc. However the
broad regexp does mean this occasionally picks up stuff that isn't
a tag. So manually amending the commit is probably a wise idea,
and so I simply decided to also leave a '--- extracted tags ---'
separator in the commit message just before the extracted tags,
which can be cleaned up manually when verifying that the tags look
correct.

v2: Drop the aliases
    Remove the temp file
    Clean up locals
    Other codingstyle cleanups
    Use '# *** ... ***' for the separator

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>