Update documentation with XPath extensions.
Change version to v0.2.0a1 until 0.2.0 release
Change .js to .json in usage examples for file parselets (fixes #3)
Change example file parselet extension from .js to .json
Merge remote-tracking branch 'origin/v0.2a1' into v0.2a1
Separate JSON file Parselet examples for CSS rules and XPath rules (fixes #4)
Visually separate configuration and execution in Python shell example (fixes #5)