Add display-related fields to emote metadata

[noodlebox]

Include some metadata to help rendering emotes better:

• width and height (should help resolve #11 for certain special cases of irregularly sized emotes)
• available sizes (necessary to implement #10 properly)
• render options, such as whether to avoid scaling, or pixelate when scaled up (also helpful for #11)
• custom styles (e.g. FFZ's Halloween emotes like StabZ)