Add optional feedback on the cost of the PoW

[guilhermelawless]

Very much optional.

Needed config file parameters:

• Peak power usage of CPU/GPU ( in W )
• Cost of kw/h in $ (to keep it simple).
• display_real_cost flag so that this is optional

Compute as: power_usage[W] * sum(pow_time)[s] * kwh_price[$/kWh] / (3600[s/h] * 1000[W/kW]

Example: 150 * 300 * 0.20 / ( 3600 * 1000 ) = 0.0025$