add fallback_buffer_dir option

[tastytea]

This adds an option to change the root directory to the buffer directory, if the file is outside of a project.

I often edit files outside of projects while my cwd is somewhere else. Telescope and nvim-tree can be convinced to open the buffer's directory without changing this plugin, but for :edit, :write and running shell commands in the same directory there is no alternative.

[musjj]

Looks like this is the fourth(?) PR for this kind of feature now:

• https://github.com/ahmedkhalf/project.nvim/pull/56
• https://github.com/ahmedkhalf/project.nvim/pull/62
• https://github.com/ahmedkhalf/project.nvim/pull/88

I guess it's up to @ahmedkhalf to decide on which one to merge

[tastytea]

Ah sorry, i only saw #56, which would add the current dir to the list of projects, if i'm not mistaken.