Escape more special characters in regexps

In particular, start="\\\@<!\[" is the wrong way to define the start of
a region: it will work for \[, but also for \\[, which is erroneous.

if \\ and \[ are matched first, then you don't need to guard against a
preceding backslash.

Addresses https://github.com/guns/vim-clojure-static/issues/30