Added drush mm-meta-copy and mm-meta-update.

3 new drush functions to manipulate metadata of lots of assets in 1 run.

drush -h mm-meta-copy
drush -h mm-meta-update
drush -h mm-reset-counts

for example:
drush mm-meta-update 1 dc.subject 'Subject foo'
drush mm-meta-copy 1 dc.subject dc.type
drush mm-reset-counts 1