Report battery discharge/charge rate

[DiegoGuidaF]

It would be really nice for laptop users being able to get a graph of the battery discharge rate. I think it is a really important metric for laptop users (at least it is for me).
I have been able to hack ytop in order to show it (change battery percentage to discharge rate), however I have never programmed in rust and a few extra features would be really difficult for me to implement, such as:

• Increase interval from 60s (value used for bat. percentage) to same as
  cpu usage.
• Change text color depending if charging/discharging.
• Show time_to_empty/time_to_full values depending on state.
• Manage the plot color accordingly to previous status.
• Allow user to choose between battery % or data rate via an extra parameter:
  · -b: Battery percentage (current implementation)
  · -br: Battery energy rate.

If you find you won't have time to do this I could implement it myself and do a PR for it, as long as you review it and ensure it is not a mess I would be happy to try it :)