Add support for msgctxt to i18n

[drd]

This is a bit of a blast from the past, but some internet meandering led me to realize that the work I had done at Idealist many years ago to add support for a new i18n:ctxt attribute (think pgettext) never landed. The tests still appear to pass :)

Trac ticket: https://genshi.edgewall.org/ticket/580

[hodgestar]

@drd I know it's been awhile, but would you mind merging master into this branch and then I'll review.

Apologies for this issue taking seven year! 🙈

[spaceone]

@hodgestar FYI: in github PR's you can push to the branch if allow edits from maintaines is enabled for this PR. You can simply rebase the branch on master, there are only 3 simple conflicts.

[hodgestar]

@spaceone Thanks! I was actually hoping @drd could take a quick look again so that the code will have had two recent sets of eyes on it when it gets merged. If you're happy to review it with me, that would be awesome (but if not, no worries).

[hodgestar]

Left a couple of small suggestions and I need to see if the tests are passing after my merge of the recent changes, but thank you for dusting this off and submitting it. Much appreciated.

[hodgestar]

@drd Thank you for taking the time to resurrect this. I've cleaned it up a bit and am merging.