[Ur] New release

[Adam Chlipala]

I'm trying out a new release method, based on the GitHub feature of the 
same name.  You can find this release at:
     https://github.com/urweb/urweb/releases/tag/20180616

CHANGELOG for this release:

- New feature to cache files stored in the database as blobs, via the
   'filecache' .urp directive
- New .urp directives: 'mimeTypes' and 'file' (new long form)
- New HTML pseudo-tag: <cradio>
- New HTML tag attributes: 'oninput', 'onscroll', 'title', 'size'
- New standard-library functions: 'List.findM' and 'List.existsM'
- New '-help' command-line option for compiler
- Remove insecure function 'Basis.crypt' (which didn't seem to have any 
users)
- Selenium-based automatic testing
- Bug fixes and improvements to documentation and error messages