Add "Monthly" Trigger to ScheduledTask

[ephos]

Request:
ScheduledTask does not allow the ability to do a monthly trigger, with this functionality missing it makes it difficult to use DSC to create a task that runs on a specific day each month. This resource lacks feature parity with it's PowerShell/GUI alternatives.

Use case example:
Run a task on the 3rd Monday of each month.

Additional Notes:
This can be worked around somewhat by the suggestions in #173 but having to manage doing this through logic external to the resource seems counter productive to using something declarative like DSC.

[johlju]

Can't this be accomplished with the WeeksInterval parameter and have the ScheduleType set to Weekly?

How would you otherwise set a monthly trigger using the PowerShell commands?