Blockquotes

(this depends on pull request #21)

This pull request implements blockquote tags, denoted `[<blockquote>]` and `[</blockquote>]`. I went for this syntax rather than the more conventional one of prefixing every quoted line with a `>` because this syntax offers a very direct translation from the current `[<html>]<blockquote>[</html>]` usage, and the things blockquoted on the cfl wiki tend to be large bodies of text, and prefixing every line would be very tedious. It also mirrors the syntax for similar block-level elements, like `[<java>]` and `[<html>]`.

This fixes REVIKI-372.