Release 0.5.0
### Added
- (breaking) `Attributes` events for dangling attribute sets (#56).
- Implement TryFrom<&str> for `Attributes`, allowing to use the attributes
parser on its own (#56).
- More examples in documentation (#56).
- `AttributeValue::new`, `AttributeValue::default` (#56).
### Changed
- (breaking) Replace `Render::push_borrowed`, `Render::write_borrowed` with
`RenderRef::push_ref`, `RenderRef::write_ref`.
- (breaking) Turn map-like opaque `Attributes` into wrapper of Vec<"AttributeElem"> (#56).
- (breaking) Preserve comments and duplicate values in attributes (#55, #56)
(but still do not render).
- (breaking) Block attributes followed by a blank line are not attached to the next block.
### Fixed
- Remove extra whitespaces in attribute values (#56).
