Add option to show only virtual text of the highest severity

[ryu-ichiroh]

I think people who the settings below don't want to show multiple virtual text of diagnostics, because it is only for diagnoses of lower severity that the line is broken.

let g:lsp_diagnostics_virtual_text_wrap = "truncate" 
let g:lsp_diagnostics_virtual_text_align = "right"

For example, don't want to see warning message 'a' is declared but ....

[hrsh7th]

I think it's useful but I also think it should be let g:lsp_diagnostics_virtual_text_severities = [1,2,3...].