Add "stage" for Arabic diacriticization using Rababa

[ronaldtse]

The Rababa gem is used for Arabic script diacriticization.

In order to use Interscript for Arabic transliteration, the text goes through these stages:

1. A text like قطر is diacriticized (diacritics added, called the pointed form) into قِطْرَ
2. The text of قِطْرَ is transliterated by an Arabic map

In reverse transliteration, the following steps:

1. The Arabic transliteration is reversed into a diacriticized Arabic string (pointed Arabic)
2. If desired, the pointed Arabic is dropped into the unpointed form (no diacritics)

We need to implement a stage that allows Arabic diacriticization.

Maybe there needs to be a processing option to do this - user-specified determination of pipeline stages:

• e.g. "diacriticize => bgnpcgn-ara-..."

[webdev778]

PR #735