Prevent generated header collisions, less naively.

> This is a rework of an earlier version of this code.

The automatic header ID generation code submitted in #125 has a subtle
bug where it will use the same ID for multiple headers with identical
text. In the case below, all the headers are rendered a `<h1
id="header">Header</h1>`.

  ```markdown
  # Header
  # Header
  # Header
  # Header
  ```

This change is a simple but robust approach that uses an incrementing
counter and pre-checking to prevent header collision. (The above would
be rendered as `header`, `header-1`, `header-2`, and `header-3`.) In
more complex cases, it will append a new counter suffix (`-1`), like so:

  ```markdown
  # Header
  # Header 1
  # Header
  # Header
  ```

This will generate `header`, `header-1`, `header-1-1`, and `header-1-2`.

This code has two additional changes over the prior version:

1.  Rather than reimplementing @shurcooL’s anchor sanitization code, I
    have imported it as from
    `github.com/shurcooL/go/github_flavored_markdown/sanitized_anchor_name`.

2.  The markdown block parser is now only interested in *generating* a
    sanitized anchor name, not with ensuring its uniqueness. That code
    has been moved to the HTML renderer. This means that if the HTML
    renderer is modified to identify all unique headers prior to
    rendering, the hackish nature of the collision detection can be
    eliminated.