Fix main class, allows loading on options pages

[JayWood]

Pretty much a rewrite of the main initializing class. Used CMB2 as a staple for this sort of thing. The previous main class wouldn't allow us to use the plugin on an options page. It now does.

• Version bump to 1.2.5
• Using singleton pattern now
• Now firing only on cmb2_init, not after_setup_theme

@jtsternberg I know you originally had a hand in here, so if you get time to review that'd be amazing.

[jtsternberg]

This will completely break the loader functionality as you are not actually using the PRIORITY constant which is what makes most of the magic work.

You'd be better off using the newer loader that I have in the wds-shortcodes plugin: https://github.com/WebDevStudios/WDS-Shortcodes/blob/master/wds-shortcodes.php#L112-L118
(this is all based on wp-lib-loader, and the version in this plugin is old)

It hooks into the first available hook, depending on when the file is loaded, which is what you're looking for.
https://github.com/WebDevStudios/WDS-Shortcodes/blob/master/wds-shortcodes.php#L112-L118

[JayWood]

@jtsternberg awesome, thanks for the info