ERROR-log on symbolic links

filepath.Walk does not follow symbolic links.
There's no easy fix for that outside of Go, so the best we can do for now is to give notice to the end user by ERROR log statements.

This commit also fixes a related panic situation in GenerateTemplateNameFrom when the layout dir was a symbolic link.

Fixes #283
Make Urls in menus respect CanonifyUrls

Fixes #519
Revert "Make Urls in menus respect CanonifyUrls"

This reverts commit 3ccb3979028aafb3e6cc16d47daa77a1fdea11fa.
Commented commands package
Trigger an editor after `hugo new`.

- Trigger permanently with NewContentEditor in config.{toml,yaml,json}.
- Trigger on an individual basis with --editor.
Use current working dir as WorkingDir instead of hugo executable's dir
Provide (relative) reference funcs & shortcodes.

-   `.Ref` and `.RelRef` take a reference (the logical filename for a
    page, including extension and/or a document fragment ID) and return
    a permalink (or relative permalink) to the referenced document.

    -   If the reference is a page name (such as `about.md`), the page
        will be discovered and the permalink will be returned: `/about/`
    -   If the reference is a page name with a fragment (such as
        `about.md#who`), the page will be discovered and used to add the
        `page.UniqueID()` to the resulting fragment and permalink:
        `/about/#who:deadbeef`.
    -   If the reference is a fragment and `.*Ref` has been called from
        a `Node` or `SiteInfo`, it will be returned as is: `#who`.
    -   If the reference is a fragment and `.*Ref` has been called from
        a `Page`, it will be returned with the page’s unique ID:
        `#who:deadbeef`.

-   `.*Ref` can be called from either `Node`, `SiteInfo` (e.g.,
    `Node.Site`), `Page` objects, or `ShortcodeWithPage` objects in
    templates.

-   `.*Ref` cannot be used in content, so two shortcodes have been
    created to provide the functionality to content: `ref` and `relref`.
    These are intended to be used within markup, like `[Who]({{% ref
    about.md#who %}})` or `<a href="{{% ref about.md#who %}}">Who</a>`.

-   There are also `ref` and `relref` template functions (used to create
    the shortcodes) that expect a `Page` or `Node` object and the
    reference string (e.g., `{{ relref . "about.md" }}` or `{{
    "about.md" | ref . }}`). It actually looks for `.*Ref` as defined on
    `Node` or `Page` objects.

-   Shortcode handling had to use a *differently unique* wrapper in
    `createShortcodePlaceholder` because of the way that the `ref` and
    `relref` are intended to be used in content.
Documentation for `ref` and `relref`.

- Rejigged the weight of the extras/ content for the new crossreferences
  page.
- Used the new {{</*…*/>}} format for documenting highlighting and to
  prevent a warning about the missing `fig` shortcode.