Originally I tried to bodge in a handcoded parse transform and it
did not go very well.  This time, we simplify the case statement
in lager.erl and use this simplified version in the parse transform
after getting the AST from the Erlang compiler.

Previously, code would only look up the default sink's
loglevel, not the referenced sink name's loglevel.

In other words, if lager_event's loglevel was set to NONE,
it squelch all events from all sinks, and conversely if
a sink's loglevel was set to NONE, it would be silently
ignored.

If a sink is referenced in a call, but is not configured
(by being a running process), then emit an
{error, {sink_not_configured, Sink}} tuple.

Throw {error, bad_config} if we find one during
handler startup.