Add the 'alert' extension to Pandoc standard markdown

[radcool]

The present issue is to formally ask if it is possible to add the capabilities of the alert extension currently supported by commonmark_x and gfm to Pandoc's (standard) markdown, as per jgm/commonmark-hs#132 (comment).

[ryanjm]

Documentation says it is supported but it isn't (anymore).

[jgm]

At the moment it's supported but only for commonmark and variants.

[radcool]

Documentation says it is supported but it isn't (anymore).

The way I read the instructions, it only says it's supported for GFM and commonmark.