Additional Gitlabs Markdown support

[tlrand]

FSNotes already supports a lot of what the Gitlabs extensions covers (and very well too!). The two additional ones that I would actually use are:

1. Alerts - Warnings, Tips, etc in info blocks.
2. [Multi-line block quotes] (https://docs.gitlab.com/user/markdown/#multiline-blockquote) - The ability to fence a block quote using a >>> delimiter. It is easier to use, reads better as text and would allow for block level attributes if FSNotes ever grows those.