Make oil extension behave like default configuration

[abeaufays]

Prior to this change, the oil extension would show an empty status line except for the file name in lualine_a.

This change make the status line behave like the default one, and sets the filename / lualine_c as the path to the shown directory.

Arguments in favor of this change:

• Keeps the status line consistent while browsing a project
• Mode is an important information inside a Oil view of a directory
• The default behavior shows the filepath as Oil:///[absolute path to file]

Caveats:

• It reads lualine.config.get_config() during extensions resolving, which is kind of a smell and may mess up other configs / make the extensions order important
  • 💭 Maybe there is a way to cleanly pass the user config to the extensions ?
• It assumes the filename is in lualine_c, which could have been changed by user.

Oil directory status line before this change:

Oil directory status line after this change:

📓 If this change is accepted, I'd like to make the filename configurable. I'm thinking of choosing the line to write in, and choose between relative / absolute / custom function