Use custom blackfriday renderer to only add mention/hashtag links in normal text

[blackle]

fixes #783. blackfriday lets you specify your own renderer, which gives us more control over where we insert the hashtag/mention links. see: https://github.com/russross/blackfriday/blob/v2/html.go#L508

basically, blackfriday.Text signals raw text output, and blackfriday.Code signals code block output, so we can avoid making the conversion there.

caveat: posting something like:

_#hashtag_

doesn't turn #hashtag into a hashtag, but

_#hashtag #hashtag #hashtag_

converts all three, so I think the code that generates the tag list needs some work in the markdown case.

[tsmethurst]

Nice fix! Can you add a couple test cases for this in the text package?

I think the code that generates the tag list needs some work in the markdown case

mm, sounds like it 🤔 we should open another issue to figure out these cases, will you do it or should I?

[blackle]

added more tests, also opened #789 for the other issue