Add caseInsensitive option to parser builder

[curvedmark]

Fix #108

[curvedmark]

The second commit makes the parser builder try case-insensitive comparison only if the literal actually contains alphabet. Character sets are also covered.

I simply use /[a-z]/i to test alphabet, which should be suffice. If the language is case-insensitive and also need to support international characters, the author can manually append is anyway if she doesn't want slight performance penalty.

[dmajda]

Sorry I didn’t get to look at this, closing as obsolete now. See also my comment at #108.