Disable early validation of colorscheme option

Adding early validation of options in ReadSettings() caused a regression:
colorschemes registered by plugins via config.AddRuntimeFile() stopped
working, since ReadSettings() is called when plugins are not initialized
(or even loaded) yet, so a colorscheme is not registered yet and thus
its validation fails.

Fix that with an ad-hoc fix: treat the "colorscheme" option as a special
case and do not verify it early when reading settings.json, postponing
that until the moment when we try to load this colorscheme.

internal/config/colorscheme.go

@@ -55,6 +55,14 @@ func InitColorscheme() error {
 	c, err := LoadDefaultColorscheme()
 	if err == nil {
 		Colorscheme = c
+	} else {
+		// The colorscheme setting seems broken (maybe because we have not validated
+		// it earlier, see comment in verifySetting()). So reset it to the default
+		// colorscheme and try again.
+		GlobalSettings["colorscheme"] = DefaultGlobalOnlySettings["colorscheme"]
+		if c, err2 := LoadDefaultColorscheme(); err2 == nil {
+			Colorscheme = c
+		}
 	}
 
 	return err