[roleboard] allow some basic entry format args

[Chovin]

allow some basic rolecall entry format args, like customcommands does. things like..

1. last member to join {role1.last_member_joined.mention}
2. last role joined {last_role_joined.mention}
3. member count {role1.member_count}
4. maybe even a list of the last x members to join

implementation suggestions:

1. only update it every-so-often. don't want to be in danger of hitting rate-limit for heavy users of this cog.
2. only update it if necessary, if changes are needed