Changed Markdown quote syntax to <blockquote>.

Pinboard no longer accepts Markdown in link descriptions due to an XSS vulnerability ([see this tweet from 2011](https://twitter.com/Pinboard/status/22436355472625664)). This pull request changes the behavior (and the name) of the `quoteSelection` flag to wrap the description in `<blockquote>`s instead of prepending it with `>`.