rc spell: Allow storing a language in an option
rc spell: Allow storing a language in an option

This commit adds a `spell_lang` option that will be used by the
`spell` command when no language is passed to it directly.

Setting a buffer/window local option allows using `:spell` seamlessly
across several buffers, written in different languages.

casimir/kakoune,alexherbo2/kakoune,lenormf/kakoune,lenormf/kakoune,danr/kakoune,danr/kakoune,jkonecny12/kakoune,lenormf/kakoune,casimir/kakoune,mawww/kakoune,occivink/kakoune,occivink/kakoune,danr/kakoune,jkonecny12/kakoune,occivink/kakoune,Somasis/kakoune,mawww/kakoune,alexherbo2/kakoune,Somasis/kakoune,casimir/kakoune,occivink/kakoune,Somasis/kakoune,alexherbo2/kakoune,danr/kakoune,jkonecny12/kakoune,jkonecny12/kakoune,mawww/kakoune,alexherbo2/kakoune,mawww/kakoune,casimir/kakoune,Somasis/kakoune,lenormf/kakoune