I searched the source code of Go, and found no usages of
"== (reflect.Value{})" and "!= (reflect.Value{})". I think
it's more idiomatic to use "IsValid" to check a zero Value.
New config "tab" allows user to set a string instead of literal tab
(e.g. "... ").
New config "source-list-tab-color" allows that string to be colored.
This builds on top of the `updateColorScheme` change
