Add `eyebrowse-mode-line-indicator`

[ven]

Hi,

Could you add a predefined "workspace-mode-line" or "workspace-list" segment that uses eyebrowse-mode-line-indicator? This makes eyebrowse totally usable as a vimtabs replacement.

It's not too bad currently, it only requires this (in dotspacemacs/user-init):

  (spaceline-define-segment workspace-list
    "Gives a list of workspaces, akin to vim tabs"
    (eyebrowse-mode-line-indicator))

  (setq spacemacs-spaceline-additional-segments '(workspace-list))

but I often see people (as well as myself) asking for visual cues as to which workspaces are open