Allow per-document CSS override files

Allow the addition of document-specific CSS by creating a CSS file and storing it alongside the Markdown, eg. running conversion on "sample.md" in directory "work_directory" per:

```
work_directory
| sample.md
| sample.css
```

will apply the styles found in sample.css as the last step of loading CSS.

Also:
- Added a setting to disable the feature ("allow_css_overrides") in case of collision
- Reworked the getCSS function slightly so that overrides will also apply with non-default base css configured in settings