Using StackExchange/pagedown instead CodeMirror editor

[sergeysviridenko]

Hello!

• Type: new feature
• Link to issue: #88

In raising this pull request, I confirm the following:

• [x] I have read and understood the Contributing Guidelines
• [x] I have checked that another pull request for this purpose does not exist
• [ ] I wrote some tests for this PR

Small description of change: Using StackExchange/pagedown instead CodeMirror editor

Thanks

[sergeyklay]

Thank you