Make space after ``` customizable

[phst]

59a4986 introduced a space after ```, which is valid but uncommon (e.g. https://guides.github.com/features/mastering-markdown/#GitHub-flavored-markdown) doesn't have it. Would be great if this behavior were customizable.

[cjsut]

Additionally, the CommonMark specification allows arbitrary meta-data after the opening code fence:

The first word of the info string is typically used to specify the language of
the code sample ... However, this spec does not mandate any particular
treatment of the info string.

I personally use this to specify how a particular block of code should be rendered, for example ("this code block is erroneous" or "this code block is pretty okay").

A minor change to markdown-regex-gfm-code-block-open allows specification of metadata after the language without de-fontifying the entire code block, but better coloring and fontification of the metadata would be nice.

[jrblevin]

The development version (and possibly earlier ones to some extent) supports such arbitrary "info strings".