Navbar items

Branch contains setter and getter methods for the following navigation items:

• source_code (add remove button to show RMD code or as hyperlink)
• social_links (links to social media)
• method add_to_navbar: adding custom link to the navigation bar.

PROBLEM: The method add_to_navbar is not finished, because the creation of the following YAML structure required by flexdashboard is missing (probably not possible with the package ymlthis) (https://rmarkdown.rstudio.com/flexdashboard/using.html#navigation_bar)

---
title: "Navigation Bar"
output: 
  flexdashboard::flex_dashboard:
    navbar:
      - { title: "About", href: "https://example.com/about", align: left }
---