v1.3.2: Auto-purge; Retain config; etc.

🚀 Feature

• Purges the NGINX User Cache if a post/page is published or updated (#3)

💅 Polish

• Add a Settings link to WP's Installed Plugins page (#9)
• Add a warning if the plugin is activated but not configured (#4)
• Colour the button backgrounds (traffic light coming in v1.3.3) re: UX (#9)

🐛 Bug Fix

• Expand the size of the config fields to show API key (#9)
• Retain existing config values during a WP Plugin Update (#8)