Add ability to provide callbacks in styles array.
Add ability to provide callbacks in styles array.

Added the ability to provide callbacks as a style.  If a callable is
passed as the value of a style option stapler will invoke the callable
while also injecting in the current uploaded file object and the current
instance of Imagine.

riyan8250/stapler,flaxandteal/stapler,sonuku/stapler,aozisik/stapler,CodeSleeve/stapler,oschettler/stapler,AndreasHeiberg/stapler,InMoji/stapler,simudream/stapler,neerajgoyal12/stapler