Only start the highlights worker thread when we need to
Only start the highlights worker thread when we need to

Also make sure the user has run `make init-highlights` by checking if
the coffee executable is in the right location.

tbrowder/doc,MasterDuke17/doc,stmuk/doc,perl6/doc,dmaestro/doc,tbrowder/doc,jonathanstowe/doc,dmaestro/doc,gfldex/doc,jonathanstowe/doc,antquinonez/doc,dmaestro/doc,stmuk/doc,jonathanstowe/doc,gfldex/doc,jonathanstowe/doc,dmaestro/doc,gfldex/doc,stmuk/doc,stmuk/doc,antquinonez/doc,MasterDuke17/doc,perl6/doc,gfldex/doc,tbrowder/doc,MasterDuke17/doc,jonathanstowe/doc,antquinonez/doc,gfldex/doc,perl6/doc,antquinonez/doc,MasterDuke17/doc,tbrowder/doc,perl6/doc,perl6/doc,dmaestro/doc,stmuk/doc,antquinonez/doc