Start work on highlighting search results for nijel/weblate#42

So far this only knows about source strings.
Also highlight search results in context field (#42)
Capitalization fixes for search term highlight (#42)

Not profiled in any way. Hopefully not as bad as using `re.findall` to
spot variations and then using `str.replace`.
Merge pull request #269 from ChrisOelmueller/highlight-search-results

Highlight search results