Add mercury language syntax highlighting.
Merge branch 'mercury' of github.com:mucaho/highlight.js into mercury

Conflicts:
	AUTHORS.en.txt
	docs/css-classes-reference.rst
	src/test.html
Adding mucaho to AUTHORS.ru.txt
Mentioning Mercury in CHANGES.md.
Tuning inter-relevance between SML and Mercury.
More relevance work for Mercury:

- don't illegalize ::
- add `:-` as a relevance booster
Reformatted mercury.js for consistency.