c-family.kak: add plain c highlighter and detection
c-family.kak: add plain c highlighter and detection

Give priority to extension over detected mimetype, as
file often takes c++ files to be c files.

occivink/kakoune,Asenar/kakoune,occivink/kakoune,Asenar/kakoune,Somasis/kakoune,jkonecny12/kakoune,casimir/kakoune,flavius/kakoune,mawww/kakoune,jjthrash/kakoune,casimir/kakoune,flavius/kakoune,mawww/kakoune,flavius/kakoune,danr/kakoune,lenormf/kakoune,flavius/kakoune,jjthrash/kakoune,alexherbo2/kakoune,lenormf/kakoune,alpha123/kakoune,alpha123/kakoune,jjthrash/kakoune,Somasis/kakoune,alexherbo2/kakoune,alpha123/kakoune,alpha123/kakoune,danr/kakoune,occivink/kakoune,ekie/kakoune,jkonecny12/kakoune,ekie/kakoune,jkonecny12/kakoune,alexherbo2/kakoune,mawww/kakoune,danr/kakoune,alexherbo2/kakoune,lenormf/kakoune,danr/kakoune,Asenar/kakoune,Asenar/kakoune,mawww/kakoune,Somasis/kakoune,Somasis/kakoune,lenormf/kakoune,casimir/kakoune,casimir/kakoune,jkonecny12/kakoune,ekie/kakoune,jjthrash/kakoune,ekie/kakoune,occivink/kakoune