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}
```

Or, we make a larger change that checks for this particular flag being enabled through a configuration option, defaulting to on. (Older blogs will need to make one change to their `config.*` file to maintain the current behaviour, and it could also *potentially* be on a per-page basis. Maybe.)