add 'x-github' support

[torkus]

https://github.com/search?q=%22X-Github%22++extension%3Atoc&type=Code&ref=advsearch&l=&l=

• [x] toc, x-github is detected in toc files and added to the source-map-list
• [x] gui, x-github data is available from the 'other sources' column and 'source' context menu
• [x] review
• [x] update CHANGELOG