Add fetch command

[Kidlike]

I would like to have the git fetch command available through homesick.
This way I can do a homesick fetch to see if I am behind from the remote.

I have a homesick-all function in my bash functions that will run the provided homesick arguments for all castles.

So then I could do a homesick-all fetch and immediately see if any of my castles need updating.

[Kidlike]

I just realized that the new version has a -all switch for some commands, so hopefully my bash function will eventually become obsolete :)

[JCook21]

As mentioned in the comments on #135

[JCook21]

As mentioned in the comments on #135 this can be achieved through the exec_all command.

[Kidlike]

Closed as per #135