Support case insensitive tags

[joaomvfsantos]

Hello, before all thank you very much for this library. I would like to ask you if it is possible to add case insensitive tags recognition. For example:

Style("sup")

should match both tags below:

<sup>Superscript</sup>
<SUP>Superscript</SUP>

However only the lowercased tag is matched. At the moment, to overcome this I have to either duplicate the Style for an uppercase condition, or do a string replace.