Added maximum number of email to read and multiple mailbox names to read with getmail management command

[eshafik]

This PR:

• Added named argument [optional] to read one or multiple mailbox from getmail command.
  Example: python manage.py getmail -mb mailbox1 mailbox2 or python manage.py getmail --mailboxes mailbox1 mailbox2

• Added named argument [optional] to read maximum number of emails from mailbox. It can be used from getmail management command -
  Example: python manage.py getmail -mr 10 or python manage.py getmail --max_read 10

• Updated docs/topics/polling.rst file to guide the users for this changes.

This changes have been used in one of our live project which handles thousands of email per day. It might be helpful to other also.

[Fahad-Md-Kamal]

Seems really an important control and necessary configuration.

[Aninda07]

This is very useful feature. We had to need this control for our project also.

[Shadiul]

Very helpful feature to have, would be very productive for the projects I'm currently working on.

[coddingtonbear]

I'm very curious to hear if anybody genuinely would find something like this useful -- I don't understand what the use case is for this and would like some insight.

That being said: I have to say that it's more than a little suspicious that the three ❤️s/comments that this PR received were not only the only ❤️s/comments that it has ever received, but that they were all posted within one hour of one another, and just a couple hours after the PR was posted. So: I'm at this point assuming that the ❤️/commenters are just members of your social network, @eshafik, and I'm inclined to assume their comments are not genuine.

[ptav]

I think the limit option at least is certainly useful. But please use a name more in keeping with django and python common practice (i.e. not a 2-letter arbitrary abbreviation). Maybe --limit or --max-messages