This commit allows using the <semicolon> expansion in commands, instead
of `\;`.
It makes commands look more elegant, and prevents new-comers from
falling into the trap of using <a-;> without escaping the semicolon.
For clojure.kak and haskell.kak, set extra_word_chars on the buffer
and not the window, so that the buffer's word db is updated correctly
for word completion.
Document this.
Also, minor refactor of clojure.kak.
Closes#3108
