Gollum Integration

[jmatsushita]

Would allow a Gollum wiki to be a data source/sink. It would offer support for Github/Gitlab and Realms.io wikis.

• Frontmatter and other (block level) Metadata would be added and removed using 3 way diff approaches.
• Dealing with reorganisation of wiki pages into folder structures in contentascode would be a good difficult case.
• Dealing with importing content snippets from external source would be another interesting case.

[jmatsushita]

Ikiwiki is also interesting