HTML Editor Type & Export Options

[hreikin]

Description

This PR is a follow up to #13. More work is currently needed before this will be ok to merge, in its current state this PR's changes include:

• Restructures the toolbars
• Adds a HTML editor
• Converts editor content when a different editor type is chosen
• Configure markdown > HTML conversion extensions (pymdownx: Github style configuration as suggested in FAQ)
• Syntax highlighting in the document preview
• HTML syntax highlighting
• Document theming
• Template system (Jinja2 - basic, no template inheritance as of yet)

Changes still to be made:

• Toolbar buttons apply appropriate syntax when in Markdown or HTML mode
• Edit the full page HTML before export functionality
• Export options

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested

Still To Be Updated
Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Test A
• Test B

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings