Add color support and style configurability

Color support for glkterm. Makes it possible to configure a color pair for each textbuf/textgrid style.

Defines a `OPT_USE_COLORS` option which can be disabled for (n)curses implementations that don't implement color support.

Provide a `-style` command line option to load a 'style sheet' specifying the curses style for every glk style.

Two example style files are provided.