Bug Fixes and New Features

### Issues Resolved
- Fix for #246 progressbar CSS issues.  Update the CSS overall and fix the 1px off issue.
- Add `enable_pygments` option in settings to allow Pygments to not be used.  This is to solve frustrations that some users have who want to use a Javascript syntax highlighter. #248
- Update the default `github.css` to the latest version from https://github.com/sindresorhus/github-markdown-css. Also update the github Pygments style to the last Pygments update that Github had made before they abandoned using Pygments.  Also add the old Pygments Github style as `githubclassic`. #252
- Add support for translating paths to relative paths (in relation to HTML output). #247

### Other Fixes
- Fix CSS bug where code blocks in Python Markdown output had no spacing between them

### New
- Remove obsolete `b64` extension (as Markdown Preview uses a global base64 converter).
- Remove obsolete `admonitionicon` extension as this is now done with pure CSS.
- Upgraded some 3rd party extensions:
    - githubemoji: upgrade to latest (no functional changes)
    - headeranchor: Add itself without needing changes to toc extension
    - magiclink: Even better regex rules to fix some corner cases
    - tasklist: No functional changes
    - github: Now includes superfences by default
    - progressbar: Now requires title to be quoted (no longer optional).  Inline settings support is removed in favor of using inline [attr_list](https://pythonhosted.org/Markdown/extensions/attr_list.html) support (new).  Syntax now requires only one equal sign, but can take any number now.  addclasses and levelclass settings renamed to add_classes and level_class.
- toc extension reverted to its original form as it no longer conflicts with headeranchor
- Plain html option now also strips html comments.  Also re-written to be better and more straight forward.
- base64 conversion now ignores content in html comments
- absolute and relative path conversions can now handle paths with trailing html fragments, like when it is desired to jump to a page and jump to a header on that page => `path/to/myfile.html#some-header`.  The trailing fragment will now be preserved.
- Add superfences extension which allows for nesting of fenced code blocks in lists, admonitions, blockquotes, etc.
- Add UML flowchart and sequence diagram support through superfences.  Requires flowchart.js and sequence-diagram.js which are provided (and their dependancies through CDNs; flowchart.js has no CDN so it is directly included).
- Update test file with new progressbar example and new flowchart and sequence diagram examples.
- Update readme in relation to changes.
- Better admonition styling that doesn't clash with other element's styling.