Check for a valid application layout before deployment when linting

This change blocks deployment when the application directory specified doesn't match one of the following conditions:

1. We have `server.R` and `ui.R` in the base directory,
2. We have `server.R` and `www/index.html` in the base directory,
3. We have `index.html` within the base directory,
4. We have an `.Rmd` file within the base directory (not so sure on this one)

Comments appreciated!
Nitpicky, but you already `toLower()`ed, so you can just use `.*\.rmd$` for the regex if you wanted.