Adding support for "data-bfi-processed-class" option

The new option, "data-bfi-processed-class", allows to define a class name that is added to every processed file input element. This allows to make sure that each input file element only gets processed once. 

This could be useful if multiple file input elements are injected into page's DOM dynamically using Javascript and bootstrap-file-input functionality needs to be applied to newly injected input file elements only.