config: add [colors].dim0-7

This allows you to configure custom colors to be used when colors are
being dimmed (`\E[2m`).

It is implemented by color matching (just like
bold-text-in-bright=palette-based); the color-to-be-dimmed is matched
against the current color palette.

If it matches one of the regular colors (colors 0-7), the
corresponding “dim” color will be used.

If it matches one of the bright colors (colors 8-15), the
corresponding “regular” color will be used (but *only* if the “dim”
color has been set).

Otherwise, the color is dimmed by reducing its luminance.

The default behavior, i.e. when dim0-7 hasn’t been configured, is to
dim by reducing luminance for *all* colors. I.e. we don’t do any color
matching at all. In particular, this means that dimming a bright color
will *not* result in the corresponding “regular” color.

Closes #776

foot.ini

@@ -68,6 +68,8 @@
 # alpha=1.0
 # foreground=dcdccc
 # background=111111
+
+## Normal/regular colors (color palette 0-7)
 # regular0=222222  # black
 # regular1=cc9393  # red
 # regular2=7f9f7f  # green
@@ -76,6 +78,8 @@
 # regular5=dc8cc3  # magenta
 # regular6=93e0e3  # cyan
 # regular7=dcdccc  # white
+
+## Bright colors (color palette 8-15)
 # bright0=666666   # bright black
 # bright1=dca3a3   # bright red
 # bright2=bfebbf   # bright green
@@ -84,9 +88,18 @@
 # bright5=fcace3   # bright magenta
 # bright6=b3ffff   # bright cyan
 # bright7=ffffff   # bright white
+
+## dimmed colors (see foot.ini(5) man page)
+# dim0=<not set>
+# ...
+# dim7=<not-set>
+
+## The remaining 256-color palette
 # 16 = <256-color palette #16>
 # ...
 # 255 = <256-color palette #255>
+
+## Misc colors
 # selection-foreground=<inverse foreground/background>
 # selection-background=<inverse foreground/background>
 # jump-labels=<regular0> <regular3>