Release 0.7.0

Breaking Changes:

• Style::new() now takes an Into<Cow<'static, str>> instead of
  Into<String> and returns stylist::Error instead of String when
  encountering an error.
• Style::create() now takes Into<Cow<'static, str>> for class prefix
  and css string and returns stylist::Errorinstead of String when
  encountering an error.
• Style no longer implements ToString.

Other Changes:

• Added a new API YieldStyle.
• Added theming examples.
• Styles are now cached by default.
• Fixed a Bug where .a-class-name is after @media would cause parser
  to return an error.
• Added Docs.
• Removed Unnecessary Clones.
• Optimised for Performance.