add function knitr_fig_path

Only cleaning intermediate R files on demand.

1.  Use site.root for root path if set, otherwise use site.github.url.
2.  Display extra files for workshop-template repo as well as for lessons.

1.  Moving all YAML library usage into 'util.py' via utility function.
2.  Wrapping 'import yaml' in error handling to report what library to install.

1.  Converting error reporting.
2.  Simplifying checking logic.
3.  Refactoring metadata loading.
4.  Modifying lesson checker to reflect refactoring.

Closes #47.

Closes #55.

1.  Splitting commands to clean intermediate R files into separate target.  These files must be committed to the repo, so they shouldn't be deleted under normal circumstances.
2.  Modifying intermediate file generation to create fig/rmd-* rather than fig/swc-rmd-*, since this template is used by other carpentries (not just SWC).

`generate_md_episodes()` is modified such that datasets storred into
_episodes_rmd/data/ needed to knit the episodes, are copied into
_episodes/data/, and are made available at the root of the site.

Only try building R files if there are some