Add support for passwordcommand in email_imap.

Hi. I wrote a patch be able to run a process to get the password for the email_imap exension (since I didn't want to put my password in the config file). I can now run a little script to use gpg to extract the password. To achieve this I had to have a general purpose process class which I also added. I was wondering if you would be willing to merge. 

To use I just do this:

...
        {
            "module": "email_imap",
            "config": {
                "passwordcommand": "/path/to/script/that/runs/gpg",
                "username": "username@gmail.com"
            }
        }
...


Foroevrig regner jeg med at du er Norsk :-)