Add a way to omit computing colors for terminal
Refactor generate_gradients.py
Reverse the gradient in battery segment

Closes #910
Document how to create gradient colors
Use battery_full and battery_empty for displaying hearts

Red hearts for battery full are still more optimal in case white hearts are for
empty (though I do not understand why they have to be red). Cannot agree about
per cents though: red is better for empty then white is if there are no related
parts to compare.
Add two more colors to white_red gradient cterm colors

Picked up with

    tools/generate_gradients.py 231 223 216 209 196
Regenerate gui colors for white_red gradient

Command used to generate colors:

    tools/generate_gradients.py -C 231 255 223 216 209 202 196 --weights='1 1 1 1 1 1 3'
Default line_percent_gradient to (light|dark)_green_gray gradients

dark_green_gray gradient was generated with

    tools/generate_gradients.py 70 247

. It is used for non-insert modes in the current window. light_green_gray
gradient was generated with

    ./tools/generate_gradients.py 148 250

. It is used for insert mode in the current window.
Reverse JSON output and add trailing comma

Makes it easier to copy-paste output to colors.json.