Removed ale_virualtext_prefix from debugging since it's not requried for
the functionality to work.
Sorted debugging info to make the list easier to navigate/diff.
- Add g:ale_virtualtext_cursor boolean to enable/disable it
- Add g:ale_virtualtext_prefix to configure what prefix to use (default:
'> ')
- Requires neovim 0.3.2's unreleased API `nvim_buf_set_virtual_text`