feat: add custom walker support

[tobim]

As the title says.

Example: Format a git repo + a specific submodule:

walk = "repoPlusSubmodule"

[walker.repoPlusSubmodule]
command = "bash"
options = [
  "-c",
  '''
    submodule="path/to/submodule"
    {
      git ls-files --cached --others --exclude-standard --full-name
      git -C "$submodule" ls-files --cached --others --exclude-standard --full-name \
        | sed "s#^#$submodule/#"
    }
  ''',
]

Example: Format a pijul repo:

walk = "pijul"

[walker.pijul]
command = "pijul"
options = ["ls"]

[jfly]

@tobim, thanks for the contribution! I am supportive of this change, but haven't had a chance to read the code yet.

I wanted to warn you that #694 touches some of the same code you've touched here. It may land before this PR, apologies if the conflicts are annoying to deal with.