support default colors in non-`TB_OUTPUT_NORMAL` modes (#41)

support default colors in non-`TB_OUTPUT_NORMAL` modes (#41)

`TB_DEFAULT` is now defined as a bitwise attribute similar to
`TB_BOLD`. previously it was defined as 0 which made it impossible to
use default colors in non-`TB_OUTPUT_NORMAL` modes.

for convenience and limited back-compat, 0 is still interpreted as
`TB_DEFAULT` in `TB_OUTPUT_NORMAL`, `TB_OUTPUT_216`, and
`TB_OUTPUT_GRAYSCALE`, though `TB_DEFAULT` may be used as well.

in `TB_OUTPUT_256` and `TB_OUTPUT_TRUECOLOR`, 0 is black, so
`TB_DEFAULT` and `TB_TRUECOLOR_DEFAULT` respectively must be used if
a default color is desired.