Enable descriptive header IDs.

Enable blackfriday.EXTENSION_AUTO_HEADER_IDS to generate the name of the
header ID from the text in the header. Works for prefix and underline
headers.

__WARNING__: This change is not backwards-compatible, so it should *probably*
be put under a configuration option. That is, the existing behaviour of Hugo is
to use #toc\_*n* for header IDs. Now, a header will match its contents.

```markdown
  # A Header
```

Under the old model, that would generate:

```html
  <h1 id="#toc_1">A Header</h1>
```

Under the new model, that will generate:

```html
  <h1 id="#a-header">A Header</h1>
```

The automatically generated TOC will use the correct values regardless, but it
may be advisable to explicitly specify the old #toc\_*n* model as part of the header:

```markdown
  # A Header {#toc_1}
```