Ignore headers in code block

[syohex]

This is related to #27 , ardumont/markdown-toc#13
CC: @Wilfred, @ardumont

In original code, statements which looks like markdown header such as shebang in code block are handled as markdown header. For example, markdown-toc generates table of contents form markdown headers which is retrieved by markdown-imenu-create-index.

 # Heading

 ## subheading

 ```
 foo
 # bar
 baz
 ```

 ## another subheading

markdown-toc generates following table of contents with original code. #bar in code block is handled as markdown header.

<!-- markdown-toc start - Don't edit this section. Run M-x markdown-toc-generate-toc again -->
**Table of Contents**

- [Heading](#heading)
    - [subheading](#subheading)
- [bar](#bar)
    - [another subheading](#another-subheading)

<!-- markdown-toc end -->

markdown-toc generates following table of contenst with this change.

<!-- markdown-toc start - Don't edit this section. Run M-x markdown-toc-generate-toc again -->
**Table of Contents**   
- [Heading](#heading)
    - [subheading](#subheading)
    - [another subheading](#another-subheading)
<!-- markdown-toc end -->

[jrblevin]

Thanks for this patch! Perhaps your new markdown-collect-fm-code-blocks function could be used for font locking to also fix #33?

[syohex]

Perhaps your new markdown-collect-fm-code-blocks function could be used for font locking to also fix #33?

I'll check.