Do print two spaces if no battery is available

This ensures a constant length of the output, which is nice if the output is right-aligned, and you have several tmux instances nested from different machines (some with battery, some without).
Implement better mapping from values to ticks
Add option to specify custom colors
Show newest measure on right-most bar (no avg)
Extract smoothing to own method
Remove tabs
Ensure presence of a non-zero value in @chartvals

This avoids a division by zero later.
Allow floating point values for threshold