Gradient-colored bars

[hfaran]

Two kinds:

• The entire bar is colored y=f(x) where x is the current value
• The gradient is mapped across the bar, so each character has color y=f(x) where x is the character's value relative to max_value

REALLY don't want numpy to be a dependency (no offence to numpy but it's just too damn huge), so should either find a gradient function in another library, license numpy's or roll my own variation.