SPIKE: Handle language hints in fenced code blocks

This patch implements handling for GFM-style language hints on fenced code
blocks. The following input:

    ```bash
    echo 'hai'
    ```

Becomes:

    {code:bash}
    echo 'hai'
    {code}

This patch also changes the input parser to "GitHub Flavored Markdown" instead
of the default "Kramdown". This parser retains information such as language
hints from fenced code blocks.

Ref. jedi4ever/markdown2confluence#6