mailbag to destination

[nkrabben]

Is your feature request related to a problem? Please describe.
I often need to create a bag in a different location than my working directory.

Describe the solution you'd like
I'd like to mailbag to a destination like --destination path/to/staging/mailbags/ or -m /path/to/staging/mailbags/mailbag.name

Describe alternatives you've considered
I could move it after the fact
mailbagit ... -m mailbagname | mv mailbagname path/to/staging/mailbags/

[gwiedeman]

Draft implementation in moving_options-226-227. Just needs fixing tests, more testing with companion files, and updated docs.

[gwiedeman]

Implemented by #229