feat: allow specification of maxlength from textarea element

[garyforsterio]

Hey,

Thanks for all the great work on this. I needed the md-maxlength attribute on the textarea type so went ahead and added it to the source file and also the unminified dist file.

I tried running npm run build but it seemed to add some additional changes which I hadn't made, so to be safe I thought it should just made this basic PR.

Cheers, Gary