Allow multiple dirs and subdirs

[strboul]

It can be useful to check the git status of the multiple dirs.

git-substatus ~/dir1 ~/dir2

It's also good to check the multiple sub-dirs (but cannot check dirs and sub-dirs at the same time).

git-substatus ~/dir1 ~/dir2 --subdirs

Think a bit more about how dirs and subdirs should be printed on the terminal.

Extra use case:

If you git track the system folders like /etc (via a tool like etckeeper) or ~/.config,
there can be a script written to get a git status of those folders such as:

system_git_status() {
  git-substatus --subdirs=\
    /etc, \
    "${XDG_CONFIG_HOME:-~/.config}", \
    "${XDG_DATA_HOME:-~/.local/share}" \
    --include-hidden
}