Add feature to parse text in the lesson definition.

Backward compatible changes which allow to process lesson definitions which request some marker (*e.g.* `-_-`) to be replaced with text generated with an R expression upon loading the lesson.

Written to fix an issue due to locale-specific translation of character strings with `strptime`.

Fixes swirldev/swirl#201.

Lesson specification should be used as in odeleongt/swirl_courses@53ff493b19d74d991752345dbf467e5a050827f3.