Add option to work along with -r such that some fields won't be relative but absolute

When looking at SHOW GLOBAL STATUS output with pt-mext it would be very nice to see absolute values and not the deltas for certain fields which are not "ever-increasing" like most counters, for example Innodb_current_row_locks, Innodb_buffer_pool_pages_free, Threads_running, Threads_cached and others which are "current state" and not "count since dawn of times". Something like --no-relative-fields=coma,separated,list,of,fields would be an easy way to pass the info (and the option would imply --relative)