Add with_tail parameter for parsley:str() extension
Add parsley:strip() extension function to strip some characters,
just like Python .strip() for strings
Support user-defined extension functions and add context arg to .parse()

The context is the first parameter passed to user extension functions
Revert test for empty extracted content
Python3 syntax fixes and remove some debug strings
Add smart_strings=False when compiling XPath expressions
Add more HTML block elements to parsley:html() extensions
Update changelog
Rename some functions. Started reworking extraction logic (WIP)
Refactor a bit how matching elements are extracted
Introduce overridable _default_element_extract() method to
XPathSelectorHandler to customize default element extraction
More refactoring
Add tests for parsley:strip() extension