Add a character-specific class to enable kerning combinations

It can be useful to kern every instance of certain letter combinations. With these extra classes, you can now apply more broad css rules. For example, to consistently tighten the space between the characters "A" and "W", you could use the css rule `.char-a + .char-w { margin-left:-2px; }`

* use array to build class list
* initialize array within loop
* use regex to check item and only add classes for letter characters