add support for more style attrs and 64-bit attrs (#56)
probably should have broken this into multiple commits but it was all
inter-related:
* deprecate TB_OPT_TRUECOLOR in favor of TB_OPT_ATTR_W which defaults
to 16, but can be 32 (truecolor) or 64 (truecolor + additional
styles). for back compat, TB_OPT_TRUECOLOR sets TB_OPT_ATTR_W to 32
if not already set.
* add TB_BRIGHT, available in TB_OUTPUT_NORMAL only.
* add TB_DIM and TB_INVISIBLE. TB_INVISIBLE is available only when
TB_OPT_ATTR_W=64.
* add TB_STRIKEOUT, TB_UNDERLINE_2, TB_OVERLINE, available only when
TB_OPT_ATTR_W=64. these are hard-coded caps because they don't
appear to be in terminfo.
* deprecate TB_256_BLACK and TB_TRUECOLOR_BLACK in favor of
TB_HI_BLACK which can be used in any of the hi-color modes.
* deprecate all TB_TRUECOLOR_* style attributes. users can use
TB_BOLD, TB_UNDERLINE, etc in truecolor mode now.