Add color themes properties

[TristanCacqueray]

The goal of this issue is to enable the user to provide their own CSS values for the ones hard-coded here:

re-ansi/src/Ansi.re

Lines 37 to 56 in 6e07d47

	// Convert a 4 bits color code to its css color: https://en.wikipedia.org/wiki/ANSI_escape_code#3-bit_and_4-bit
	let fourBitColors = (code: int): option(string) =>
	switch (code) {
	| 00 => "black"->Some
	| 01 => "red"->Some
	| 02 => "green"->Some
	| 03 => "yellow"->Some
	| 04 => "blue"->Some
	| 05 => "magenta"->Some
	| 06 => "cyan"->Some
	| 07 => "white"->Some
	| 09 => "initial"->Some
	| 10 => "grey"->Some
	| 11 => "#DA2647"->Some
	| 12 => "#87FF2A"->Some
	| 13 => "#FFF700"->Some
	| 14 => "#5DADEC"->Some
	| 15 => "#FF3399"->Some
	| 16 => "#8DD9CC"->Some
	| 17 => "white"->Some

.

In particular, we need to enable the user to provide their own initial value for the code 9, as documented in #3

To do that, we could:

• add an optional fourBitColors function property to the React component, or
• define a color palete type property, and modify the existing fourBitColors function to use that instead.

When color themes is implemented, it might be good to also provide default themes, such as dark, light, and their solarized version.