Allow options for header anchor links

[JGallardo4]

Current behavior when header_links is set to true is to add an empty anchor link with href set to the header's id and place this inside the header but before the other elements:

## Section 2. Example with a `code span`

Renders as:

<h2 id="section-2-example-with-a-code-span">
  <a href="#section-2-example-with-a-code-span"></a>
  Section 2. Example with a
  <code>
    code span
  </code>
</h2>

---

It would be nice to have options avaiable for setting the anchor tag inside the header but after the other elements:

## Section 2. Example with a `code span`

Renders as:

<h2 id="section-2-example-with-a-code-span">
  Section 2. Example with a
  <code>
    code span
  </code>
  <a href="#section-2-example-with-a-code-span"></a>
</h2>

---

One could have the option to set the anchor tag as a wrapper over the header's contents. This is what Mozilla Developer Network's docs site does, and it's really nice. See an example article

## Section 2. Example with a `code span`

Renders as:

<h2 id="section-2-example-with-a-code-span">
  <a href="#section-2-example-with-a-code-span">
    Section 2. Example with a
    <code>
      code span
    </code>
  </a>
</h2>

---

Additionally, it would be good to be able to specify the content to be rendered inside the anchor tag (when not set as a wrapper):

## Section 2. Example with a `code span`

Renders as:

<h2 id="section-2-example-with-a-code-span">
  <a href="#section-2-example-with-a-code-span">PERMALINK 🔗</a>
  Section 2. Example with a
  <code>
    code span
  </code>
</h2>

[gettalong]

I'm open to a pull request that modifies the header_links option to support the described use-cases (e.g. allowing the values true, false, before, {before: LINK_TEXT}, after, {after: LINK_TEXT}, and around). The implementation should be backwards-compatible.