Post generator

I think it would be cool if the Jekyll command could also generate posts for you, ie `jekyll post 'My new Post`. This would scaffold out the title, date, and optional markdown for you in the correct format. Creating a post by hand isn't too difficult, but having a generator to do this for you would be a nice touch. 

I'm not sure if I hit on every edge case in the testing, so please let me know if anything needs to be added. Also, are there any plans to raise exceptions when there are generator conflicts?  Right now you can keep creating Jekyll projects within Jekyll projects and other generator commands and I'm wondering if this is a cause for concern. Nevertheless, good work with Jekyll!