Add a Command header to the AMI Mixmonitor action.

This fixes a parsing error that occurred during the processing of
the AMI action. The error did not result in MixMonitor itself
misbehaving, but it could result in the AMI response not giving
correct information back.

The new header allows for one to specify a post-process command
to run when recording finishes. Previously, in order to do this,
the post-process command would have to be placed at the end of
the Options: header. 

Patches: mixmonitor_command_2.patch by jhardin (License #6512)
........

Merged revisions 412048 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@412050 f38db490-d61c-443f-a65b-d21fe96a405b