not support setext-style head

[oldratlee]

Sample:

<!-- TOC depth:6 withLinks:1 updateOnSave:1 -->
- [Bar H1](#bar-h1)
    - [Bar h2](#bar-h2)
<!-- /TOC -->

Foo H1
=========

Foo H2
--------------

# Bar H1

## Bar h2

Fail to create toc for Foo H1 and Foo h2.

my env info

markdow toc package version: `0.2.1`
atom version: `0.153.0`
OS: OS X `10.10.1`

PS:

• setext-style head