Pybars4

- Added support for segment literals (`[foo bar]`)
- Fixed a bug related to newlines in the middle of tags
- Added support for single-quoted string literals
- Added the `lookup` helper
- Added support for quoted partial names

- Added support for negative integers
- Added support for parent acccess to data elements (`../`)
- Added support for keyword args being passed to partials
- Added explicit `_parent` data access
- Added `root` data support
- Added `index`, `key`, `first` and `last` data access
- Added support for object attributes within the `each` helper
- Added Python 3 support