Add some convenience queries to equipment/equipment-category routes

[Redmega]

Would be nice to query something like:

• /api/equipment-categories/weapon?weapon_category=simple
• /api/equipment-categories/weapon?properties=finesse

I'm sure there are others that folks would find useful.

[bagelbits]

Some of this is probably related to 5e-bits/5e-database#397

However, the properties would be a nice addition. I'm here for it.

[bagelbits]

@Redmega Do you think this would be more useful on the /api/equipment route? Putting it on the /api/equipment-categories/weapon route seems odd.

[Redmega]

Hmm... I could see it being useful there. I think the initial thought was that properties like weapon_category were mainly on weapons, but I'm sure if we look at the data there might be similar properties that are shared between equipment in general?

[bagelbits]

I've been considering adding an equipment_categories field that has an array of all categories that apply to an equipment. Then we don't have 3-4 different fields like we do right now. I assume that would make it a bit cleaner.