Day of the week enhancement

[ranjithkommula]

At present the library don't have the support for dow.

Hash (#) '#' is allowed for the day-of-week field, and must be followed by a number between one and five. It allows you to specify constructs such as "the second Friday" of a given month. For example, entering "5#3" in the day-of-week field corresponds to the third Friday of every month.

[robfig]

I would be open to a pull request, assuming this feature can fit into the current model and comes with comprehensive tests.

[ChaimHong]

@robfig same needs