Support RPC & CTL plugins.

For example, if you want to use mr.laforge, write the supervisor section as follows:

[supervisor]
plugins =
    mr.laforge

rpcplugins =
    laforge mr.laforge.rpcinterface:make_laforge_rpcinterface

ctlplugins =
    laforge mr.laforge.controllerplugin:make_laforge_controllerplugin