Replace variable_*(), theme_*_variable() with D8 CMI and added excludes to config.

I've added a comprehensive patch that achieves the following:

• Created the config directory and added all settings to bootstrap.settings.yml
• Added breakpoints to config/bootstrap.breakpoints.yml
• Added config/schema/bootstrap.schema.yml schema definition
• Removed all calls to variable_*() with calls to the D8 CMI Config service
• Removed all calls to theme_*_setting() with calls to the D8 CMI Config service. While this still works, it will eventually be depreciated/removed.
• Moved the exclude CSS and JS functionality to the D8 CMI Config service
• Removed the "bootstrap_" prefix to all theme variables as there is now namespacing, so the prefix is no longer needed. I updated all uses of the Bootstrap variables to reflect this.
• One instance of variable_get was within a check for the jQuery Update module. This is no longer needed so I removed the entire check. Updated README.txt.
• Cleaned up bootstrap.info.yml file. Moved type variable to the top for performance reasons: https://drupal.org/node/2198695

Needs some review. Thanks guys!

Thanks @ryan.armstrong!

Committed 0345c0c to 8.x-3.x:

Issue #2190877 by ryan.armstrong, rabbit_media: Replace variable_*(), theme_*_variable() with D8 CMI and added excludes to config.