We were passing the separators for the `show-whitespaces` highlighter
verbatim. This was problematic in case one wanted to use, spaces,
quotes or `%` as separators since the resulting kakoune configuration
would be invalid.
According to kakoune's docs, the separator has to be one character
long, so we can use a simple rule for escaping them. It is possible
that people has been working this around by passing, e.g. `"' '"` as
separator in order to get a space (i.e., escaped explicitly by the
user), so we just let longer strings be used verbatim.
PR #1357
