Support for .mokuro format in mokuro reader

[blakedgross]

Mokuro updated to have it's own "mokuro" format with html now being the legacy format. Their web reader recommends using the mokuro format per their 0.2.0 update, but jidoujisho only supports selecting .html for the manga reader.

[bpwhelan]

I might consider contributing to help support the new format, but likely just to get the "volume up and down to move pages" functionality. That's the main thing mokuro reader in jidoujisho is missing for me atm.