add support to dotnet try Markdown for specifying certain settings as file-wide defaults

[jonsequitur]

Being able to specify --project, --file, and --package options at the level of the whole Markdown file will reduce the verbosity and repetition caused by having these annotations specified on each fenced code block.

The page-level settings could be specified using YAML frontmatter (http://assemble.io/docs/YAML-front-matter.html), which some Markdown parsers recognize.