The option is now used as a fallback when detection by extension fails. Some
scripts like `base/mail.kak` and `base/html.kak` still rely heavily on it.
Initializing the `formatcmd` variable in the default language support
scripts created a dependency to the `formatter.kak` script, which we do
not want. Examples of such cases are when users haven't loaded the
`formatter.kak` script, or when Kakoune runs its test suite and
selectively loads some language scripts for testing purposes.
