Bug fixes, Tests, Updates and feature flags

This is one of the largest releases yet thanks to new contributors and me getting around to doing more work!

Things in this version:

• Syntax tests: there is a new syntest example for running Sublime Text syntax tests
• Bug fixes: there's a ton of bugs fixed in this release, mostly found via the syntax tests. These mostly affected certain syntaxes which pushed/set multiple contexts at once.
• Updated packages: The Sublime packages have been updated to the latest version
• Feature flags: there's now Cargo feature flags for disabling some parts of syntect if you don't want unnecessary binary and dependency bloat.