Allow for custom ITokenizers

[mikegoatly]

Currently the index supports plain text and Xml content during tokenization. The TokenizationOptionsBuilder should allow for a custom ITokenizer to be provided, e.g.

TokenizationOptionsBuilder.WithCustomerTokenizer<T>(Func<TokenizationOptions, T> constructionDelegate) where T: ITokenizer;

builder.WithCustomTokenizer(options => new CustomTokenizer(options));

[mikegoatly]

Closing this because #27 is going to supersede the reason for doing this.