Added concept of "letter"

This makes (letter --> word) work in the same way as (digit --> integer).

Although the constructed regex will now be slightly different for anything containing words, this is fully backwards compatible.
Version bump to 0.1.5
Version bump to 0.1.5
Added explicit '#letter' and '#word' tests

It's very difficult to make good tests for regex that actually only test ONE THING. The alternative is to go a bit over-the-top, like I probably have here.
Correction
Correction
Merge pull request #10 from tom-lord/patch-1

Added concept of "letter"