htmlparser5 and PostCSS

Added

• Support ES6 Classes (see #79)

Changed

• Use htmlparser2 to parse XML, HTML, and SVG
• Use PostCSS Safe Parser to parse CSS, Less, and SCSS

Fixed

• Various PHP issues, for example multipe inheritance, thanks to @pelatx
• Use Class icon for PHP Class definitions
• Don't break on PHP return types (see #80)