initial support for nullable R Markdown format options
initial support for file options
store document-relative file paths in YAML
improve usability of R Markdown nullable format options
passthrough for R Markdown options not understood in the dialog
hide formatting options if format is not from a known template
show "null" rather than "~" for YAML null values
make R Markdown format edit a dynamic command
fix restoration of default values; improve file option behavior
fix bug: JS exception determining self-containment of unknown formats
transfer applicable options on in-place format switch
Merge remote-tracking branch 'origin/master' into feature/advanced-rmd-options

Conflicts:
	src/gwt/src/org/rstudio/studio/client/rmarkdown/model/RmdTemplateData.java
Merge pull request #97 from rstudio/feature/advanced-rmd-options

Feature/advanced rmd options