JNDI Lookup plugin support

Currently, Lookup plugins [1] don't support JNDI resources.
It would be really convenient to support JNDI resource lookup in the configuration.

One use case with JNDI lookup plugin is as follows:
I'd like to use RoutingAppender [2] to put all the logs from the same web application context in a log file (a log file per web application context).
And, I want to use JNDI resources look up to determine the target route (similarly to JNDI context selector of logback [3]).

Determining the target route by JNDI lookup can be advantageous because we don't have to add any code to set properties for the thread context and JNDI lookup should always work even in a separate thread without copying thread context variables.

[1] http://logging.apache.org/log4j/2.x/manual/lookups.html
[2] http://logging.apache.org/log4j/2.x/manual/appenders.html#RoutingAppender
[3] http://logback.qos.ch/manual/contextSelector.html