Add new filename path option

[ptdewey]

Thanks for the great plugin!

I was finding it mildly confusing to tell which file I'm in at any given time in a Rust project I've been working on recently due to the presence of many 'mod.rs' files. However, I prefer my status line to be quite minimal, ideally showing only the filename. In this case, I thought it would be helpful to dynamically add the parent directory to the filename component when multiple buffers are open with files of the same name.
As such, I decided to add a new path option to lualine to accomodate this behavior.

Example:
Directory structure:

src
├── mod.rs
└── parser
    └── mod.rs

Relevant lualine config:

lualine_c = {
     { "filename", path = 5, padding = 1, },
 },

With one of the mod.rs files open, lualine will look like this:

When opening the other mod.rs file, lualine will now look like this: (for parser/mod.rs)

And this for src/mod.rs:

Deleting either buffer will cause the component to revert to showing only the filename again.