Add yaml_writer.R to accommodate a simpler authoring workflow

This pull request incorporates the yaml_writer.R script, which was originally composed by @WilCrofter. Any virtues of this new approach should be attributed to @WilCrofter. I accept credit for any and all flaws.

Some changes/additions have been made, including `testit(from, to)`, which is compatible with the `swirl/fromto` development branch (not yet merged). Also, lesson meta data is stored in `options()` (see `set_swirlify_options()` in yaml_writer.R, so that it is globally available. This prevents the user from having to pass this information as arguments to the authoring functions (`txt()`, `qmult()`, etc).

Eventually, the code added here will be more tightly coupled with existing swirlify features. However, it works well as is and will serve our immediate needs by allowing for fast prototyping of new swirl content.