Support XGG base directory specification for user configuration resolution

[rpdelaney]

Context

In #263 we added support for .mdformat.toml as an on-disk configuration file.

I would like to revisit the possibility of also reading fall-back configuration from $XDG_CONFIG_HOME.

platformdirs provides tools that can assist with this in a cross-platform way.

Proposal

The best approach I could think of is to start with the lowest priority configuration files first and work down, applying changes as we go. That way, defaults can be over-ridden by $XDG_CONFIG_HOME/mdformat.toml (or whatever we call it) but project-specific settings still take priority over any other on-disk configuration.

Descending priority would look something like (subject to negotiation):

1. $HOME/.mdformat.toml
2. $HOME/.config/mdformat.toml
3. $XDG_CONFIG_HOME/mdformat.toml
4. /.mdformat.toml
6. /path/to/repo/.mdformat.toml

Tasks and updates

• Confirm general strategy for config resolution
• Confirm prioritization of different paths where we might find a config
• Implement!