Support icons for navigation menus

[gadenbuie]

Supports an additional data attribute syntax to provide icons for navigation dropdown menus.

Previously, we supported icons only on page menu items, which are provided with the data-icon attribute. Multiple pages can be included in a single navigation menu by providing all collected pages with the same data-navmenu attribute value.

For example, these two pages are included in a nav menu title "Topics".

---
title: Issue 383
output:
  flexdashboard::flex_dashboard:
    orientation: columns
    vertical_layout: fill
---

# Pie Charts {data-icon="fa-chart-pie" data-navmenu="Topics"}

Pie Charts is accessed via a submenu entitled 'Topics'

# Info {data-icon="fa-info" data-navmenu="Topics"}

Info is also accessed via a submenu entitled 'Topics'

This change allows users to provide an icon for the navigation menu by providing the icon name in the data-navmenu-icon attribute. The first usage of navmenu must have this attribute for it to be added correctly.

---
title: Issue 383
output:
  flexdashboard::flex_dashboard:
    orientation: columns
    vertical_layout: fill
---

# Pie Charts {data-icon="fa-chart-pie" data-navmenu="Topics" data-navmenu-icon="fa-map-signs"}

Pie Charts is accessed via a submenu entitled 'Topics'

# Info {data-icon="fa-info" data-navmenu="Topics"}

Info is also accessed via a submenu entitled 'Topics'

Closes #383