Add defcustom for `persp-modestring` formatting

(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.
Merge remote-tracking branch 'jleechpe/custom-modeline-format'

Conflicts:
	perspective.el