Add to course local

[Joy-El]

1. Updated documentation it bin/github-to-canvas to clarify the need for a remote public repository for --add-to-canvas option.
2. Added a new option (--add-to-canvas-local) that replicates the add-to-canvas functionality but will use paths to a local GitHub repository stored in the YAML file. This avoids the need of storing the GitHub materials in a public repository. Includes small additions to the bin/github-to-canvas and lib/github-to-canvas.rb files (all immediately following the existing --add-to-canvas option for simplicity).