add support for shorthand rgb values

[10xJSChad]

for example, "fill:#0f0" would not work with svgshift.
"fill:rgb(0, 255, 0)" would also not work but that's a bit more of a pain to fix, and also I'm pretty sure tools like svgo convert all of those to hex, so at least there's a workaround there. No workaround that I'm aware of for shorthand values though.

[10xJSChad]

shorthand hex values like "#ff0" are now supported, others like "rgb(x, x, x)" and regular color names like "red" are not.
you could have been able to work around this using svgo, but the problem there is that svgo would turn "#f00" into "red", which is not great.