Add "borders" option

[wrouesnel]

The borders config option is added which allows setting a rendered border for bar span elements. This makes them more visible when used without colors on things like e-ink displays.

An example of the effect (when used with colors removed):

The current implementation simple allows specifying a color for the borders, and hard codes a 1px bounding box. This is under the borders key.

Possibly the borders key should be more flexible and permit several input modes - i.e.

borders: true --> black borders

borders: "green" --> borders rendered as single color

borders:
  width: 4px
  color: limegreen

Yields borders with more CSS set

and then finally something more like the "colors" option which would allow per-type border setting (not sure how useful this really would be though).

Alternately this should be refactored under the colors option, although in that case I would like to do it so something like:

colors:
  borders: black
  background-color: white
  foreground-color: transparent

was a valid input option to quickly set globals.

Closes #536