Lucid is able to parse data tables that use an example table.

This commit adds support for a variation on using scenario outlines. It is
possible to have a step with a data table where the data table itself has
rows that refer to the example table of a scenario outline. In these cases
it is necessary to have the data table populated with values from the
examples table during each run of a scenario execution.
Lucid is able to parse DocString elements.

This commit allows Lucid to recognize a type of step argument known as a
doc string. The handling for this is very similar to handling data tables,
in that both are arguments to a step. This commit also makes sure that
scenario outlines which have their parameter in a doc string can have the
correct value from the examples table substituted into the doc string's
parameter.