add submodule foreach

[armcknight]

i need to be able to iterate through submodules.

the way this API is growing, with submoduleUpdate etc, makes me think that long term it'd probably be better to have a sub-enum for submodule aliases, which then has a case for each submodule command:

       git submodule [--quiet] [--cached]
       git submodule [--quiet] add [<options>] [--] <repository> [<path>]
       git submodule [--quiet] status [--cached] [--recursive] [--] [<path>...]
       git submodule [--quiet] init [--] [<path>...]
       git submodule [--quiet] deinit [-f|--force] (--all|[--] <path>...)
       git submodule [--quiet] update [<options>] [--] [<path>...]
       git submodule [--quiet] set-branch [<options>] [--] <path>
       git submodule [--quiet] set-url [--] <path> <newurl>
       git submodule [--quiet] summary [<options>] [--] [<path>...]
       git submodule [--quiet] foreach [--recursive] <command>
       git submodule [--quiet] sync [--recursive] [--] [<path>...]
       git submodule [--quiet] absorbgitdirs [--] [<path>...]
       ```

[tib]

Totally agree with the API changes you proposed. Feel free to submit a PR if you wish.