Bumped version to 0.7-beta1

Merge pull request ampproject#950 from Automattic/update/0.6.1

Bump 0.6.1

Bump 0.6.0

Bumped version to 0.6-rc2

Add back-compat for Customizer in WP<4.9

Update version and readme

= 0.5.1 =

- Fix: issues with invalid tags not being stripped out (e.g. script tags) (h/t tmmbecker, fahmi182, pppdog, seejacobscott, RavanH, jenniejj, lkraav, simonrperry for the reports).
- Fix: issues with dimension extraction for protocol-less and relative URLs (h/t ktmn for the report).

= 0.5 =

- Whitelist Sanitizer: Replace Blacklist Sanitizer with a whitelist-based approach using the AMP spec (props delputnam)
- Image Dimensions: Replace fastimage with fasterimage for PHP 5.4+. Enables faster downloads and wider support (props gititon)
- Embed Handlers: Added support for Vimeo, SoundCloud, Pinterest (props amedina) and PlayBuzz (props lysk88)
- Analytics: UI for easier addition of analytics tags (props amedina)
- Fix: parse query strings properly (props amyevans)
- Fix: Old slug redirect for AMP URLs (props rahulsprajapati)
- Fix: Handle issues with data uri images in CSS (props trepmal)
- Fix: Add amp-video js for amp-video tags (props ptbello)
- Fix: Output CSS for feature image (props mjangda)
- Fix: Fix attribute when adding AMP Mustache lib (props luigitec)
- Fix: Various documentation updates (props piersb, bhhaskin)
- Fix: PHP Warnings from `register_customizer_ui` (props jahvi)
- Fix: Coding Standards (props paulschreiber)

0.4.2

- Fix: Prevent validation errors for `html` tag (h/t Maxime2 and everyone else that reported this error)
- Fix: Handle variable name conflict that was causing content_max_width to be ignored (h/t mimancillas)
- Fix: Prevent errors when nodes don't have attributes (h/t stephenmax)
- Fix: Back-compat for 4.5 (add sanitize_hex_color function, h/t xotihcan)
- Fix: Handle gif featured images (h/t protocolil)
- Documentation updates (props troyxmccall)

Tagging 0.4.1