GFM compatibility for list/blockquote/fenced codeblock and atx header

[tsl0922]

This PR fixes the issues described in #336 #359:

A
  - b

blockquote
> text

header
# text

codeblock fenced
```
puts hello world
```

kramdown result:

<p>A<br />
  - b</p>

<p>blockquote<br />
&gt; text</p>

<p>header<br />
# text</p>

<p>codeblock fenced<br />
<code>
puts hello world
</code></p>

github result:

A

• b

blockquote

text

header

text

codeblock fenced

puts hello world

---

NOTES:

• This PR breaks an existing test: test/testcases/span/math/normal.text, which I think can be safely disabled for GFM.
• Fixes travis-ci failure due to rouge 2.0 compatibility issue Rouge 2.0 compatibility #350

[gettalong]

Thanks for the pull request! I have cherry-picked your commit, this will be in the next release.

[MattiSG]

🎊 Thanks to both of you!