Adding ability to use multiple words as prefixes to ignore

Hi, this pull request represents an amendment to the prefixes functionality, supporting multiple word prefixes using the jQuery grep function to return matching prefixes for each list element. The string is then split on the prefix. The first character of the second element in the array of the split string (afterMatch) is then chosen as the letter class.

It adds a space to any one-word prefixes prior to doing any of this using the jQuery map function for the single word prefixes to work with this approach.

I've added a demo for testing purposes.

Please consider merging this change, an older version of the library is used with this modification at the online publisher I work for. Thanks.