add number bullet

[1nhann]

test.md :

# One

```python
#!/bin/python
print("hello world")
```

## two
hello world

### Three
hello world

# Test
test

## Hello
hi

type ./cli.js test.md --number :

root@ubuntu:~/markdown-toc# ./cli.js test.md --number
  1. [One](#one)
    1. [two](#two)
      1. [Three](#three)
  2. [Test](#test)
    1. [Hello](#hello)

[1nhann]

add a parameter called number , when it is true , the number bullet will be used.