implement colorization for encoding.TextMarshaler

[hermannm]

Changes (resolves #20)

• add TextMarshaler field to Colors, defaulting to the same color as String
• apply TextMarshaler color in encoder.encodeTextMarshaler
• update null encoding in encodeTextMarshaler to use null color, like other encoder functions
• update fatihcolor.Colors with new TextMarshaler field to match jsoncolor.Colors
• add TestEncode_TextMarshaler to confirm that the color is applied properly

I believe I've gone through every use of Colors and made sure that it accounts for the new field. All tests pass on my machine.

One consideration in making TextMarshaler default to the same color as String is that this changes the behavior of the library: such values would previously have no color, but will now be colored as strings. However, I think this is more in line with the expected behavior, and since these are separate fields, users can still customize them separately as they wish.

[neilotoole]

@hermannm I pulled and tested this locally against a private project that uses jsoncolor, and it all checks out 👍

Thank you for the quality of the PR btw! I'm going to use it as an example with my own team...