Major changes to path handling

Summary of larger updates:

• Major improvements to how paths are handled by adding the build_paths() function (not exported). Now you can use a url to build to any output types except for social and html (which both require starting from the Rmd file).
• Added build_to_pdf() function (not exported) as an internal helper to build from a Rmd or html file to the pdf.

Summary of smaller updates:

• Added assert_chrome_installed() for issue #12