Set default policies

Allow for listing and setting default policies within the OTR plugin.

It could be done with the /set commands, but it makes more sense to provide nice commands for the default policies, as we already did for "per-contact" policies.

/otr policy default
-> lists the values of default policies, in the same manner as /otr policies does for "per-contact" policies

/otr policy default NAME on/off
-> sets the on/off value for the NAME default policy.

also, this will make it easier to manage additionnal default policies within the plugin.