Span things

[trefis]

#359 reloaded:

• it's closer to @dbuenzli's initial markup
• I also <span> type constructors and their parameters together
• the implementation is much cleaner.
• I've updated the testsuite!

The final result for the extreme example I was using doesn't look quite as nice:

But it's reasonable, and Daniel seems to thing the markup proposed here will be better in general, which I tend to agree with.
I'm considering including some special handling for extreme cases like the one in the screenshot, but that will wait until a later date.

I think that unlike #359 this is in a reasonable state and is ready for review.

[trefis]

The failure is only on 4.08 and because one of our dependencies doesn't build.
But since we're using beta1 in travis (which is known to be broken), I don't think it's worth investigating. (4.08 is going to be release in the coming days, potentially even today, let's just update travis at that point).

[aantron]

Thanks!

And thanks also for including the test diff with each commit.