Change !interview DATE/TIME input mechanism to be more natural.

[nehayward]

After examining all the error cases with interview, it seems like it'd be somewhat tedious to use for a new user. I remember @zpatrick was exploring using the when package. I'm thinking that same philosophy/package could be applied to !interview. I could see a new user finding it somewhat cumbersome to use, if you exclude a digit month on mistake.

It might be more natural if these weren't errors IMO:
"parse error missing month digit": "!interview rm John 3/15/2006 09:00am",
"parse error missing year digits": "!interview rm John 03/15/06 09:00am",
"parse error missing minute digit": "!interview rm John 03/15/2006 9am",
"parse error missing hour digit": "!interview rm John 03/15/2006 9:00am",

Maybe this isn't feasible and could cause errors, but I wanted to at least discuss it.

Potential new flow
!interview add NAME "natural language"
!interview add NAME next Tuesday at 2pm @uid

similar to Fantastical

[zpatrick]

I like the idea, let's definitely come back to this once we finish up https://github.com/quintilesims/slackbot/milestone/1