Additional join col

[robinsones]

Allow users to join on multiple user columns by specifying multiple columns in the by_user argument (e.g. by_user = c("course", "user").

[robinsones]

Closes issue #37

[robinsones]

Still need to make this work for funnel_start() and funnel_step()

[robinsones]

Right now the output is a joined user column (e.g. one column named "course_user"), but I'm wondering whether we should separate it back out before returning the table.