Update plugin for Pico 1.0.0

[theshka]

*[New] It is now required that you enable the plugin in config/config.php by copying the included configuration array. *[New] Default Image setting included in docs/defaults *[New] Support for Pico's standard query-string url routing and mod_rewite *[Changed] is_homepage also checks for ?index instead of just null *[Changed] Rename filename & classname to PicoATOpenGraph.php and PicoATOpenGraph, respectively *[Changed] Expanded inline documentation *[Changed] Extend from new AbstractPicoPlugin class *[Changed] Use of new hook-names *[Changed] $settings changed to $config *[Changed] Updated README.md