Add option for custom color palette and bold text as bright

[Acervans]

Added an array of strings in the config schema to input the color palette using hexadecimals or names:
It is the only way I could find to implement this in the current schema configuration file. Could be a temporary addition until the settings GUI is implemented .

Added a boolean to turn bold text's color to the bright counterparts.
An addition I personally think is useful for darker color palettes.

[flozz]

Thank you for the PR! :D

I just have an interrogation about the default value of the palette (see below).

---

Coding style: please run black nautilus_terminal to fix the coding style automatically.

If you want to keep one channel per line here:

color_rgba = Gdk.RGBA(
    color[0] / 255.0,
    color[1] / 255.0,
    color[2] / 255.0,
    1
)

just add a comma after the last argument to avoid Black putting everything on one line:

color_rgba = Gdk.RGBA(
    color[0] / 255.0,
    color[1] / 255.0,
    color[2] / 255.0,
    1,
)

[flozz]

Thank you for this PR, I will try to release a new Nautilus Terminal version by the end of the week :)