As well as ordinary `//` line-comments, Rust regards `///` comments
as documentation applying to the following item, and `//!` comments as
documentation applying to the enclosing item, so we should copy those
to new lines, too.
That means we can now have highlighters active at global, buffer, and
window scope. The add-highlighter and remove-highlighter syntax changed
to take the parent path (scope/group/...) as a mandatory argument,
superseeding the previous -group switch.
this uses the string opening regex from the c-family highlighter to prevent
highlighting the rest of the file as a string on encountering the character
literal '"'
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.
