Fix problem with list fields containing `[[]]`

Fixes #603 - thanks @xcazin!

It no longer crashes but unfortunately if you round trip a tiddler out
of edit mode and back you’ll lose any empty double square brackets.
Fix problem with parsing lists contain non-breaking spaces

Some of the time we need to treat non-breaking spaces as though they
are not spaces (regexps treat them as spaces by default).
Fix typos in docs for image parser
Implement explicit external links