Add support for field queries

Plugins can add information to uploaded data. This is not always visible in
_source. Therefore the fields parameter can be used to retrieve this data.
This fix only works if the fields that need to be retrieved are set manually
whether in the panel settings (in columns) or by activating the "Preload
Fields" option.

This fixes #1118
Signed-off-by: Marc Koderer <marc@koderer.com>