Support relative amount for tokens and shares

[matevz]

Currently, there is no simple way to empty your account apart from copying and pasting the amount and subtracting the fee. One way to solve this is to support both the absolute number of tokens to transfer or the relative share of your account:

oasis account transfer <amount|share%> <to> [flags]

CLI would compute the correct amount by querying the account balance, compute the relative amount, compute the fee and subtract the fee from the relative amount.

In the case above, one could empty the account by invoking:

oasis account transfer 100% my_cold_wallet

The relative amount could as well be implemented for deposits, withdrawals, allowances, delegations and the same for the ParaTime-layer transactions. The undelegate command could also use the relative amount of shares.