Sum-up pubkey's balances

With amount sub-cmd:

• Add ability to specify many pubkey with --pubkeys argument.
• the f() could be call as many as there is pubkeys passed as argument
• Then, at the end, pubkeys amounts must be add-up.

mentioned in commit 3268d196

mentioned in commit 7821adcf

mentioned in merge request !68 (merged)

changed title from Add-up pubkeys amount to Sum-up pubkeys amount

changed the description

changed title from Sum-up pubkeys amount to Sum-up pubkey's balances