doc: README: fix whitespace/wrapping
README: fix typos, rephrasing
Add `g:file_line_only_on_vimenter` setting

This is meant to cover the use case where you only want to use the
plugin for args during Vim startup.

Ref: https://github.com/themagicianprospero/file-line/commit/bff7f9b8a2199b836266ab50b86b9a5b072ebf72