Merge pull request #1461 from epsitec/fix-contributing

Fix CONTRIBUTING.md spelling errors and shorter lines.
Release: Version 2.3.3.
Merge pull request #1467 from am11/master

Release: Version 2.3.3
Package: Read version from a const.
Merge pull request #1468 from am11/master

Package: Read version from a const
CSS: Add text to duplicate selector warning
Merge pull request #1473 from SLaks/css-fixes

CSS: Add text to duplicate selector warning
JSON: Added completion for "format" property values
CSS: Check properties to rescue from NRE. (#147)