dfm should be included as a git submodule in dotfiles

[seth--]

Instead of having to update dfm in the dotfiles repo by hand, it should be included as a git submodule. See seth--/dotfiles for an example. It also makes it easier to update dfm for users.

To make it possible:

• .gitmodules should be added to the skip list (d341798)
• the update (and merge?) function(s) should be changed to handle the submodule
• add the need of running git submodule init and git submodule update to the documentation
• Finally, delete /bin from the dotfiles repo and putting the submodule

[seth--]

Maybe instead of changing the current functions, we could add a new one to update dfm (dfm selfupdate or something like that) and a shortcut like umis to do umi + selfupdate

[justone]

I'm not a fan of using submodules. They are difficult to update and it adds to the time to install dotfiles on a new system. Also, switching to a submodule will cause merging errors for anyone who's forked the dotfiles repo.

I think a new selfupdate subcommand is a great idea. It could update dfm either by merging from the upstream dotfiles master or grabbing the latest from the dfm repo. I made a separate bug for the new subcommand: #17.