highlight fenced code with pandoc style attributes

This PR modifies the fenced code matching pattern to allow a
language specifier inside pandoc style attributes.

The behaviour before this commit was to embed language highlighting in
code blocks with the language specifed as in github-flavored-markdown,
i.e.

    ```python
    print 1
    ```

Following this commit, the language can be specified inside pandoc style
attributes.

Pandoc markdown allows attributes on fenced code, e.g.

    ```{#id .class key=value}
    code
    ```

the order of these attributes does not matter.

One of the 'class' arguments can be a code syntax specifier, e.g.

    ```{#id .python}
    print 1
    ```

N.B. Github currently renders language specified in pandoc style
attributes if the language is the first class specified, i.e.

    ```{.python .whatever #id}
    print 1
    ```

There is a pandoc specific syntax highlighter in [vim-pandoc], but seeing as
github parses these attributes I think it makes sense to include them in here.

[vim-pandoc]: https://github.com/vim-pandoc/vim-pandoc-syntax