* Move to goldmark

Markdown rendering moved from blackfriday to the goldmark.

Multiple subtle changes required to the goldmark extensions to keep
current rendering and defaults.

Can go further with goldmark linkify and have this work within markdown
rendering making the link processor unnecessary.

Need to think about how to go about allowing extensions - at present it
seems that these would be hard to do without recompilation.

* linter fixes

Co-authored-by: Lauris BH <lauris@nix.lv>

* Rewrite markdown rendering to blackfriday v2.0

* Fix style

* Fix go mod with golang 1.13

* Fix blackfriday v2 import

* Inital orgmode renderer migration to go-org

* Vendor go-org dependency

* Ignore errors :/

* Update go-org to latest version

* Update test

* Fix go-org test

* Remove unneeded code

* Fix comments

* Fix markdown test

* Fix blackfriday regression rendering HTML block

* Add a markdown stripper for mentions and xrefs

* Improve comments

* Small code simplification

* Move reference code to modules/references

* Fix typo

* Make MarkdownStripper return [][]byte

* Implement preliminary keywords parsing

* Add FIXME comment

* Fix comment

* make fmt

* Fix permissions check

* Fix text assumptions

* Fix imports

* Fix lint, fmt

* Fix unused import

* Add missing export comment

* Bypass revive on implemented interface

* Move mdstripper into its own package

* Support alphanumeric patterns

* Refactor FindAllMentions

* Move mentions test to references

* Parse mentions from reference package

* Refactor code to implement renderizable references

* Fix typo

* Move patterns and tests to the references package

* Fix nil reference

* Preliminary rendering attempt of closing keywords

* Normalize names, comments, general tidy-up

* Add CSS style for action keywords

* Fix permission for admin and owner

* Fix golangci-lint

* Fix golangci-lint