Optimize: Widget processing

* Update: Reroute callbacks on `dynamic_sidebar_params` hook (filter) instead of `sidebars_widgets` hook (filter)

Closes #145
Merge branch 'iss-145-rt-widgets' into develop
Optimize Widget processing: Buffer output only when necessary

* Disable output buffering when widget processing disabled
* Ref: #146
Merge branch 'iss-145-opt-buffering' into develop
Update readme for 2.2.2-b1