Implement filter WIP, using logos

[Aloso]

Closes #12

Adds a filter subcommand (f for short) with advanced color editing capabilities. filter expects a list of instructions. Each instruction is either a CSS filter or a color component instruction.

Available CSS filters are:

• brightness
• contrast
• grayscale
• hue-rotate
• invert
• saturate
• sepia

These filters work exactly the same as in the CSS filter property. For example, brightness 120% will increase the brightness by 20%. Note that most CSS filters use simple algorithms in the RGB color space, and may produce inadequate results. For better results, modify the color components directly in a suitable color space.

Color components are specified with the syntax <color_space>:<component>. For example, hsl:s specifies the saturation component in the HSL color space. For RGB and HSL, the color space is optional, i.e. r is equivalent to rgb:r, l is equivalent to hsl:l, and so on.

Each CSS filter name or color component is followed by a number. This number can be negative, and it can be written in percent. For color components, the value can be preceded my an operator, which is one of

• * for multiplication (this must be escaped in some shells)
• / for division
• = to replace the value

Examples

# set the red component to 50, increase brightness by 20% and increase the HSV saturation component by 0.3
colo filter r=50, brightness 120%, hsv:s 0.3  -- orange blue 15A

# the above is equivalent to this:
colo filter "r = 50 brightness(120%) hsv:s+0.3"  -- orange blue 15A

As you can see, the format is whitespace insensitive and ignores commas. Values can optionally be wrapped in parentheses, to be fully compatible with the CSS syntax.