Add test for recps and add mention support

[christianbundy]

This resolves #2, which complains that mention-style recps are invalid, although they seem to be used in the wild. The fix was just un-commenting a line of code, so I've also added a test (and run standard --fix) as well.

[mixmix]

I think this is a bad idea to add mention-like messages in here. My suspicion is that this definition slipped in when kieran made this module perhaps because he didn't understand recps? I could be wrong.

My main question which holds me back on this @christianbundy is :

• if you put mention-style refs in the recps field, do they automatically get encrypted to the same way they do if you put just a feedId in?

My gut feeling is allowing people to do this is confusing, and I think supporting it is a bad idea. I think mentions should be in the mentions field... and recps should be simple. BUT it could just be familiarity talking. Where did you see this in the wild, and what are your feels?

[christianbundy]

Sent you a private message with some examples about where I've seen it, but in summary:

• it looks like the mixed recps array it common in the wild, about 20% of the private messages I can see have this pattern
• about 5% use the { link, name } pattern exclusively
• the other 75% have an array of strings.

I'm using this schema definition two different ways, and I think there are maybe different needs, but I'd like to prioritize the descriptive need:

• Descriptive: Before rendering a post that I've received I'm validating the schema to make sure that I know how to handle it. Right now this is throwing errors on some private messages because of the recps rule, which I'd like to be more loose.
• Prescriptive: Before publishing a post I'm validating the schema to make sure that my posts follow the expectations of the community (and our clients). I'd like for this to be as strict as reasonable, and I think my preference would be for recps to be an array of strings.

What do you think?

[mixmix]

NICE, I like that descriptive / prescriptive thing @christianbundy