Added level control for TOC and wikipedia style auto-numbering

For example having the following options: {"extras": {"toc": {"min-header-level": 2, "max-header-level": 4, "number-html-toc": True}}}

And the following input:
		# README for Blah
		
		## Introduction
		
		## The Meat
		
		### Beef
		
		##### Steak
		
		##### Burgers
		
		### Chicken
		
		### Pork
		
		#### Mmmmmmmm, bacon
		
		# At the *top* level again!?
		
The output should be:

# 1 Introduction

# 2 The Meat

## 2.1 Beef

## 2.2 Chicken

## 2.3 Pork

### 2.3.1 Mmmmmmmm, bacon