Add a battery info segment.

Default colors range from red (full battery) to white (no battery) but can be changed via the
`battery' and `battery_gradient' settings.

Default presentation is a formatted percentage string (with keyword `batt').  The `gamify' setting
changes this to a sequence of video game hearts.

Number of steps from 100% to 0% / number of video game heart icons is controlled by the `steps'
setting (default 5).