Release 0.6.0

This release introduces modularity to xtypejs, breaking it up into smaller components by extracting various, previously bundled, functionality into separate optional xtypejs extension modules. The result is a significantly smaller xtypejs library which provides the core xtypejs type checking and data validation functionality, while still allowing the flexibility to re-introduce the various other additional functionality by applying their corresponding extension modules only when these additional functionality are required. Note that this release breaks up xtypejs into several modules, but does not break the prior xtypejs API. Therefore, existing applications may only now require several modules, though most will only still require the main xtypejs module.