Support copy/save to HTML

- [x] Write the rendered HTML to the clipboard when `core:copy` is triggered on the preview view
- [x] Write the rendered HTML to the clipboard when `markdown-preview:copy-html` is triggered on the editor view
- [x] Handle `core:save-as` in the preview view which will write and open the rendered HTML to the selected file
- [x] Add context menus

Fixes #58