Custom menu links

[cjg89]

Adds the ability to specify menu link items manually using inner shortcode contents. When inner [section-menu-item]'s are present with an enclosing [section-menu /] shortcode, automatic link generation won't occur. Section menus can still be generated automatically by including a [section-menu] shortcode with no inner contents.

Manually-added menu link items can be internal (anchors) or external. rel and new_window attributes are available to modify rel and target attributes on the generated links, respectively.

Examples:

Automatic menu items:

[section-menu]

Manually defined menu items:

[section-menu]
[section-menu-item href="#"]Some anchor[/section-menu-item]
[section-menu-item href="https://www.ucf.edu/"]An external link[/section-menu-item]
[/section-menu]

Also includes a fix in the section menu init js to generate default menu settings based on the #sections-menu element's data attributes, instead of .sections-menu.

Resolves #9.