Indicate the language of a code block

[Xenonym]

Tell us about your environment

• MarkBind Version: 1.17.1

Rationale: Developer documentation sometimes includes code to do one thing in multiple languages (eg. API use in multiple languages). It can be helpful to indicate the language of a code block in the block itself.

Example: From Travis CI documentation:

[nbriannl]

Could we open this again since the PR got reverted?

[yamgent]

Could we open this again since the PR got reverted?

Yes, thanks for the reminder, just did.

[nbriannl]

I'll take this again, following the momentum of #991, I think I'll be using markdown-it-attr. Passing in attribute multiple attributes will make it verbose, but making a html-like component syntax is just as verbose. At best, if you don't use a feature, you don't need to mention its attribute.