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.

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