[Ur] New release

[Adam Chlipala]

A new release is available at:
      https://github.com/urweb/urweb/releases/tag/20190217

CHANGELOG for this release:

- Update of standard-library string functions to handle non-ASCII UTF-8 
properly
- New command-line options: -endpoints
- New .urp directive: safeGetDefault
- New Basis functions: textOfBlob, unsafeSerialized[To|From]String
- New Top functions: mapX4, foldR4
- New List functions: allM, assocAddSorted, mapConcat, mapConcatM, 
mapMi, searchM
- New ListPair functions: mapM, unzip
- New Option function: mapM
- Flycheck integration
- Bug fixes and improvements to type inference, documentation, error 
messages, and compatibility