Support configurable log format for snapshot appender

[AFaust]

FEATURE / ENHANCEMENT

As a secondary enhancement to #57 it could be a useful feature if the snapshot appender could be configured with a custom log format. This would enable inclusion of any custom MDC data or just simplify the output. One use case I could think of (which I did in the past) is using the snapshot feature for some SearchService query profiling where the details of the queries were output to CSV format in order to be used for JMeter based benchmarks (basically collecting benchmark data from a live system).
This feature would require the addition of a dialog to the "Start snapshot" action via the client-side admin.js utilities.

[binduwavell]

Instead of encumbering start snapshot, maybe there could be a config link to the right of the start button. So folks could mess with some details if they won't but can quickly perform start/stop if they are not interested in diving in.