Support Lam-Alef ligatures

[michaeldiscala]

We use the arabic-letter-connector gem with Prawn to render PDFs. One of our clients reported that the content we generated for them was very difficult to read because we were not correctly joining the Lam Alef characters. We've sent new proofs to the client for review (they'll be better able to determine if we're rendering the ligatures correctly), but I figured in the meantime I could see if this is a patch that you're interested in. Definitely open to feedback (especially since I don't speak Arabic).

--Mike
P.S. Love the Gem -- thanks so much for putting this together

[michaeldiscala]

Based on feedback from our client, I made a few changes in 2 additional commits:

1. lam-alef characters are no longer marked as allowing connections
2. Diacritic characters are now supported (they are not replaced at all -- just marked as allowing connections)
3. Standalone hamza characters no longer cause the preceding character to be in the medial form (hamzas seem to be an exception and do not allow connections from previous characters)

[michaeldiscala]

After reviewing with our client, I realized that the hamza character change was not always correct. I have added a commit to revert that change.

I'm fairly confident in the patch at this point, with its final feature set being:

1. support for diacritical marks
2. support for lam-alef ligatures