* Add headingTypeface to SpannableTheme, use a custom heading typeface in the sample app
* Add headingTextSizes
* Switching to headingTextSizeMultipliers, adding validating annotations, adding example
* Consolidate logic, add crash if header index is out of bounds
* Initialized custom extension module
* Upgraded android gradle plugin to 3.0.1
* Shaping up the custom extension module
* Added README to module
* Small improvement for IconVisitor (added color null check)