Add defcustom for `persp-modestring` formatting

This can be added independently of #20, although it uses the same `defgroup` (defined in #20).

(persp-modestring-dividers): New defcustom to allow customizing of the
modestring markers (opening, closing and dividing text)
(persp-update-modestring): Use values from new defcustom rather than
hardcoded values.