Add startsWith and endsWith for URLs.

I realise that this library is intended to be as small and light as possible - but I think these two assertions are essential for assertions on urls - redirects - domains etc, especially when considering the code+lab marriage.

Include goes part of the way, but checking a relative url when given an absolute url and checking schemes on domains are something I end up doing every day, hence this PR.

- I say URLs but there are lots of other uses (titles in html for example often share a common prefix). Let me know if there is anything I've missed.
The link target should be `#startwithvalue`.