Remove markup from synopsis.

[WardCunningham]

Although a page can specify a synopsis, we are happy to extract a substitute from the first or second item on a page. We prefer type=paragraph items but will accept any item with a text field. However, we should make some effort to remove markup from this text because the synopsis will not be further rendered outside of its original context.

Cases we should especially consider:

• internal links should be represented by their content, sans the brackets.
• external links should be represented by their link word, sans url and brackets.
• html tags should be removed from text of type=html plugins

The synopsis should be plain text. Any html escaping should be applied as a synopsis is rendered into html, not in formulation.

This logic should be concentrated in lib/synopsis.coffee which is used by both wiki-client and wiki-server.