add -sum-field option

[satta]

This PR adds a CLI option to add a derived named field to the output, summing all the numeric values of all fields matching a given regex, e.g.

$ sudo ./ethflux -sum-field 'rx_packets=rx_[ubm]cast_packets' S1 | tr ',' '\n'  | grep rx_packets=
rx_packets=1264888403

to sum up received packets across all single entries

$ sudo ethflux S1 | tr ',' '\n' | grep packe
[0]:\ rx_bcast_packets=16
[0]:\ rx_mcast_packets=188788
[0]:\ rx_ucast_packets=128654675
[0]:\ tpa_packets=0
[0]:\ tx_bcast_packets=0
[0]:\ tx_mcast_packets=0
[0]:\ tx_ucast_packets=0
[1]:\ rx_bcast_packets=0
[1]:\ rx_mcast_packets=0
[1]:\ rx_ucast_packets=128320627
[1]:\ tpa_packets=0
[1]:\ tx_bcast_packets=0
[1]:\ tx_mcast_packets=0
[1]:\ tx_ucast_packets=0
[2]:\ rx_bcast_packets=0
[2]:\ rx_mcast_packets=0
[2]:\ rx_ucast_packets=128091371
...