Add ircv3.2 batch and batch channel and query buffer playback.

This adds [batch](http://ircv3.atheme.org/extensions/batch-3.2) support for client -> znc connections.  Also if the capability is enabled by a client, then channel and query buffer playbacks will be batched.

The batch `type` for this is `znc.in/playback`.  This is vendor-specific and follows the requirements defined [here](http://ircv3.atheme.org/specification/capability-negotiation-3.1)
The `reference-tag` is an MD5 of the channel or query name.
Additionally one parameter is provided, this is the name of the channel or query which is being batched.

At present I know of no clients which support batch, however @TingPing has indicated he is likely to add support to HexChat should this pull request be accepted.
Since the `batch` capability itself isn't finalised in the ircv3 specification. Does this need to be prefixed too?