Parsing gas from float value is not always a right idea as
a transform from float is not 1-to-1.
This commit implements Fixed8Flag which parses Fixed8 value from string.
Right now a bizarre error message can occur if an address flag was not
set: `wallet contains no account for 'AFmseVrdL9f9oyCzZefL9tG6UbvhPbdYzM'`.
This bug is 10x worse if it occures in `transfer` --from flag.