add toolbar button for front matter editing; parse YAML on server
factor format option UI out of New R Markdown dialog
add dialog that presents R Markdown format options
show options from correct template (doc or presentation)
show values from front matter in dialog
initial support for writing YAML front matter back to document
add toolbar button for switching output format (disconnected)
update front matter when format is changed in toolbar menu
update editor toolbar for R Markdown format and options
show editor R Markdown toolbar options immediately after creation
Merge pull request #95 from rstudio/feature/frontmatter-editor

Add reentrant front matter editor and format switcher for R Markdown documents