This adds a new feature to Séance where reference user reactions of
emoji can automatically be proxied. Any emoji provided by the
`proxied-emoji` config value will be proxied.

This removes the ability of the reference user to react with those emoji
and is primarily assumed to be used for things like custom heart
reactions that are unique per-user, making their usage easier.

The valid config value is a comma or whitespace separated list of
unicode emoji and Discord custom emoji IDs that should be handled this
way. `*` may also be used to indicate that *all* reactions by the
reference user should be proxied.

This commit also revises the README to more clearly list the available
configuration options.

This adds support for a `!reproxy` command which makes it easy to
"reproxy" messages from a set of provided users and proxy it from
ourselves.

This feature is dangerous, and requires configuring a
`valid-reproxy-targets` list to specify user IDs that can be authors of
the message targeted to be reproxied. The reference user is always on
this list however.

Because of the danger for collision, if a command prefix is set it is
required to be used with the command to avoid multiple Séance instances
competing for a reproxy.

If the `send-typing` option is set, Sèance will send typing events on
the proxies behalf when the reference user is typing.