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 }
---

R/i2dashboard.R

@@ -9,9 +9,9 @@
 #' @slot pages A list of dashboard pages
 #' @slot sidebar Content of the global sidebar
 #' @slot colormaps A named list with color mappings.
-#' @slot source_code Either a logical value describing whether the source code should be embeded through an item in the navigation bar or a link to a URL where the source code can be found online.
-#' @slot social_links A vector with any number of the following services: “facebook”, “twitter”, “google-plus”, “linkedin”, and “pinterest”. You can also specify “menu” to provide a generic sharing drop-down menu that includes all of the services.
-#' @slot navbar_items A list of further navigation bar items.
+#' @slot source Either a logical value describing whether the source code should be embeded through an item in the navigation bar or a link to a URL where the source code can be found online.
+#' @slot social A vector with any number of the following services: “facebook”, “twitter”, “google-plus”, “linkedin”, and “pinterest”. You can also specify “menu” to provide a generic sharing drop-down menu that includes all of the services.
+#' @slot navbar A list of links in the navigation bar (see https://rmarkdown.rstudio.com/flexdashboard/using.html#navigation_bar).
 #'
 #' @return An i2dashboard object.
 #'