Add tests from the official test suite

Release 6.1.1

New features

Tests from yaml-test-suite have been added to the project and an effort has been made to improve the conformance, thanks to @am11:

• #395 Add spec test executor for yaml-test-suite
• #400 Improve YAML spec conformance by three tests
• #401 Allow scalar to have value without space (x:y)
• #403 Relax anchor names allowed characters set
• #404 Constrain DocumentEnd parsing to allowed tokens
• #406 Improve omitted keys handling

Other changes:

• Allow to save a YamlStream to an IEmitter
• Some infrastructural changes have been made to ensure that the project would build on Linux without issues.

Bug fixes

• #396 Fix missing string quotes around json serialized enums (fixes #146)
• Increase the max simple key length to 1024 and allow to configure it
• Never emit key indicators in JSON