add variable for virtual-text alignment

I just found out about virtual-text today by updating ale and it's really good but could be better :).

It could be great to allow the end user to change the text_alignment property for the virtual-text in vim.

for exemple, I had to change manually the virtualtext.vim+172 like bellow for my taste.
```vim
call prop_add(l:line, 0, {
\ 'type': l:hl_group,
\ 'text': ' ' . l:msg,
\ 'bufnr': a:buffer,
\ 'text_align': 'right',
\})
```