add new spoiler extension

[QuietMisdreavus]

This PR adds a variant of the strikethrough extension to create a new spoiler extension, for creating "spoiler" text. Since hiding text is a matter of styling it in the client, this extension just renders HTML as a <span class="spoiler"> tag.

[QuietMisdreavus]

I'm working on updating this PR. It turns out that there are multiple implementations of spoiler text. The one i implemented was based on Discord's markup. However, Reddit uses a completely different set of delimiters, and formats its spoiler text >!like this!<. However, getting this to work as a GFM extension has involved some additional issues since ! can't currently be used as a special character by an extension. I'm trying to remedy this, but it's turning out to be more complicated than i thought it would be (which was already complicated enough).

In addition, at the same time i'm fixing some of the testing scripts to adjust to the different directory layout of this fork and update some Python conventions, since i hadn't tried executing them at that time. This is allowing me to properly run all the original CommonMark and GFM regression and specification tests, which is turning out to be very helpful.

[QuietMisdreavus]

I've updated the PR to add the option to parse Reddit-style spoiler delimiters instead of Discord-style delimiters. This involved pulling in work from upstream (#29) and fixing some unrelated issues (#30), which means that this PR now depends on those other ones. I've updated the base branch accordingly.