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 }
---

man/i2dashboard-methods.Rd

@@ -67,9 +67,13 @@
 
 \item{name}{A name for the color mapping.}
 
+\item{href}{The target of the navigation item.}
+
 \item{title}{The title of the page to be added.}
 
-\item{align}{}
+\item{icon}{The item should have either a title or an icon. (See "https://ionicons.com/")}
+
+\item{align}{Optional argument that can be “left” or “right” (defaults = “right”) defining the alignment of the user-added items in the navigation bar}
 
 \item{page}{The name of the page to which add the sidebar.}