add options for a custom header, disabling highlights and showing './' and '../'

[wintermute-cell]

Hey!

This is a pull request with several changes:

• Add an option for a custom header string
• Add an option to disable highlights
• Add an option to show './' and '../' dirs (would solve https://github.com/tamago324/lir.nvim/issues/85)

Entries in the help file are included for the default/english version.

The changes are not perfect though.

• I wasn't able to properly set a highlight group for the custom header (see "FIXME: ..." tag in lir.lua)

Here is an example of what the options could be used for, a look a little similar to netrw: