After 9ad4437, directly specifying color names (instead of syntax groups)
in syntax files no longer works. In particular *.patch and *.diff files
are not highlighted, since in patch.yaml direct colors names are used.
Restore the previous behavior of GetColor (fallback to direct colors if
no syntax group found) to fix this regression, but also make some changes
in StringToStyle and StringToColor to still fix the issue which was fixed
by 9ad4437. In other words, ensure that there is no confusion between
direct colors ("red", "green" etc) and syntax groups omitted in the
colorscheme file.
* Fix default colors for unconfigured syntax groups
When GetColor is called for a syntax group not specified in the
colorscheme, it should fallback not to the terminal's default colors
(tcell.DefaultColor) but to the colorscheme's defaults (DefStyle)
which may be different from tcell.DefaultColor.
For example, if we are using micro's default colorscheme in a terminal
which uses a black-on-white theme, then dots and commas in Go files
("symbol" syntax group in go.yaml) are displayed black on a dark
background, i.e. barely visible.
* Avoid using terminal's default colors directly
If a syntax group color is set to "default" (which we have for some
syntax groups in some colorschemes), it defaults to the terminal's
default colors (tcell.DefaultColor), which is fine for 16-color
colorschemes but not quite fine for truecolor and 256-color
colorschemes which should not depend on the terminal colors.
It should default to the colorscheme's default (DefStyle) instead.
For example, if we are using micro's default colorscheme in a terminal
which uses a black-on-white theme, then "bool" type in C files
("type.extended" syntax group in c.yaml) is displayed black on a dark
background, i.e. barely visible.
