Add optional divider (bottom-border)

[IzaakWN]

Thank you for the nice plugin! I'll try to integrate it into the TauPOG website soon!

This PR adds the option to add a divider below a given menu item, e.g.:

dropdowns:
  - title: "CMS Docs"
    #icon: "/assets/CMSlogo_white_nolabel_1024_May2014.png"
    icon: "/assets/CMSlogo_black_nolabel_1024_May2014.png"
    links:
      - text: "CAT Analysis Docs"
        url: "https://cms-analysis.docs.cern.ch/"
        target: "_blank"
        border-bottom: true
      - text: "Corrections (JSONs)"
        url: "https://cms-analysis-corrections.docs.cern.ch"
      - text: "JetMET Corrections"
        url: "https://cms-jerc.web.cern.ch"
        border-bottom: 5
      - text: "JetMET TWiki"
        url: "https://twiki.cern.ch/twiki/bin/viewauth/CMS/JetMET#Quick_links_to_current_recommend"
      - text: "E/Gamma TWiki"
        url: "https://twiki.cern.ch/twiki/bin/viewauth/CMS/EgammaPOG"
        target: "_blank"
      - text: "MUO Wiki"
        url: "https://muon-wiki.docs.cern.ch/guidelines/"
        target: "_blank"
      - text: "BTV Wiki"
        url: "https://btv-wiki.docs.cern.ch/"
    ...

The current implementation allows bottom-border to be set as

• True via yes, on, true (case-insensitive), setting the border width to 1px;
• a number (integer or float), setting the border width in units of px.

It adds and extra_style, which in the future could also be used for other customizations of the style/appearance of menu items. Maybe it makes more sense to add a divider as a separate item in the links list, but the implementation would be more painful, and I think it's user-friendly enough.

Tested locally for the TauPOG website.