Anonymous: Move "say $nick" functionality to new tell command

Allowing Anonymous.say to send a message to either a nick or an
(implicit) channel through the use of first('nick', 'inChannel') changed
the behavior of the command by making it impossible for 'inChannel' to
take effect.

This meant that any previous users of the command that expected "say
some text" to send "some text" to the current channel would instead try
to send "text" to the user "some".  Depending on the value of
conf.plugins.Anonymous.allowPrivateTarget, this would result in either
an error or a strange message to a random user.

Creating a new tell command solves this issue as Anonymous.channel now
goes back to its simple 'inChannel' wrapper.

Signed-off-by: James McCoy <vega.james@gmail.com>