This commit adds the fast and CommonMark compliant Goldmark as the new default markdown handler in Hugo.
If you want to continue using BlackFriday as the default for md/markdown extensions, you can use this configuration:
```toml
[markup]
defaultMarkdownHandler="blackfriday"
```
Fixes#5963Fixes#1778Fixes#6355
Sadly, goorgeous has not been updated in over a year and still has a lot of
open issues (e.g. no support for nested lists).
go-org fixes most of those issues and supports a larger subset of Org mode
syntax.
