Express relative amounts using previous or next UD

Options could allow to specify:

• #218: history:
  • Using the UD value when the transaction was written
  • Display relative amounts using a specified UD value (UD from Q2-Q3 2019)
• balance: Using next estimated UD, to know how much our balance is reducing
• tx: allow to send using previous UD

The fist option for the history command is the most valuable and interesting usage. This one should be the first one to be implemented.

Could be implemented in the Amount() class.