Allow not making header ids lowercase

[peci1]

Code in https://github.com/showdownjs/showdown/blob/develop/src/subParsers/makehtml/headers.js#L95 and following lines always applies toLowerCase() to the header ID.

I'd like to have an option to not make the IDs lowercase (as some tools, e.g. Boostnote, do not change case of the header IDs).

[tivie]

We do lower case comparisons because some mobile browsers and older browsers, ids are case insensitive. So, in order to prevent id duplication (and weird issues), we lower case the id attribute value.

Also, Github, and most markdown implementations lowercase generated ids

[peci1]

I agree that comparing case-insensitive by default is a good idea. However, in my use-case, I need the case-sensitive IDs, thus I'm asking for having an option to turn off the conversion to lowercase.