improve user-defined config file detection

[zapashcanon]

Hi,

The idea behind this PR is to change the way the global user-defined configuration file path is chosen, to make it more standard and to follow the suitable conventions on all platforms. I used directories.

With this PR, the path is computed as follow:

• on Linux, $XDG_CONFIG_HOME/ocp-indent/config with $XDG_CONFIG_HOME defaulting to $HOME/.config/
• on macOS, $HOME/Library/Application Support/com.OCamlPro.ocp-indent/config
• on Windows, {FOLDERID_ApplicationData}/OCamlPro/ocp-indent/config ; the path of {FOLDERID_ApplicationData} can be obtained only using SHGetKnownFolderPath

directories has no dependency, except on Windows where it depends on ctypes, it works on all OCaml version from 4.07, but I haven't tested older versions.

[nojb]

Please don't add external dependencies to ocp-indent if it can be helped at all. They complicate using it in non-opam situations.

[dbuenzli]

• on macOS, $HOME/Library/Application Support/com.OCamlPro.ocp-indent/config

Configurations for unix cli tool are perfectly fine according to xdg on macOS, for one thing that's where git puts its stuff and having it in .config makes it easy to sync over different unix machines.

I don't mind if you also want to too look there but please do not change the defaults as they are now.

[zapashcanon]

having it in .config makes it easy to sync over different unix machines

You could still sync it and just do a one time command d="$HOME/Library/Application Support/com.OCamlPro.ocp-indent/" && mkdir -p "$d" && ln -s $HOME/.config/ocp-indent/ "$d".

[dbuenzli]

and just do a one time command

I'd rather suggest not to try to "fix" things that are not broken.