replace build system with generated table of contents

Gets rid of `doc/README.mdtoc` and its build script. Since GitHub.com
renders anchors for each heading, all we have to do is put a simple
table of contents into `README.md` itself, and everything will get
linked up nicely.

Pros of this approach:
* We don't have to point out to people not to edit `README.md` anymore
* We don't have to run the build script each time README gets edited

Cons of this change:
* The "chapter" numbers are lost. They were silly anyway.

`doc/mdtoc` renders a Markdown table of contents for a Markdown file.
`doc/filter-toc` filters that down to only headings after ToC.
This script can be used to easily insert ToC into the current document
when editing `README.md` with, e.g., Vim:

    :read !doc/filter-toc %

doc/mdtoc

@@ -0,0 +1,33 @@
+#!/usr/bin/env ruby
+require 'escape_utils'
+
+start_at_level = 2
+
+headers = Hash.new(0)
+
+anchor = lambda { |title|
+  href = title.downcase
+  href.gsub!(/[^\w\- ]/, '') # remove punctuation
+  href.gsub!(' ', '-') # replace spaces with dash
+  href = EscapeUtils.escape_uri(href) # escape extended UTF-8 chars
+
+  uniq = (headers[href] > 0) ? "-#{headers[href]}" : ''
+  headers[href] += 1
+
+  href + uniq
+}
+
+ARGF.each_line do |line|
+  if line =~ /^(#+) (.+?)#*$/
+    level = $1.size
+    next if level < start_at_level
+    title = $2.strip
+    href  = anchor.call title
+
+    puts "%s* [%s](#%s)" % [
+      '  ' * (level - start_at_level),
+      title,
+      href
+    ]
+  end
+end