output files directly to output_dir (html base only)
use relative path for ioslides presentation support content
fix up absolute references in images (for figures)
allow user-specified intermediates directory
write ioslides lua to output directory
use a temporary directory for intermediates when running Shiny apps
rename files_path -> files_dir
Merge pull request #93 from rstudio/feature/direct-output

Support for rendering from read-only locations