Simplify R Markdown creation experience and organize options

This change removes the format options entirely from the New R Markdown dialog. Instead, the dialog now lists the available formats, with radio buttons to choose the default.

It also contains several improvements to the format options dialog:
- Formats are now grouped into tabs, with the goal of making them easier to find and eliminating the need for scrollbars for formats with many options.
- Several new format options (mostly advanced ones) have been exposed, and UI widgets to represent those options have been added.