Specify case insensitive matching of token upon definition

[ktsivkov]

First I want to say that I really like the api of this library, it is very easy to use.

What I want to propose:

Would be great if we could specify specific tokens to be matched case insensitive.
Lets say I have a token called WHERE, would be great if it also matched where.
i.e.
Tokenizer.DefineTokens(TOKEN_TYPE, []string{"WHERE"}, true)
or
Tokenizer.DefineInsensitiveTokens(TOKEN_TYPE, []string{"WHERE"})

[bzick]

I think i's good idea. I'm looking at how to implement the idea