UI should prompt for title, not file name

I think it would be an enhancement for the UI to prompt for the post title, not the file name, and then normalize that title into `yyyy-mm-dd-post-title.md`. I've [created a fork](https://github.com/grahampcharles/jekyll-post) to do that and would be happy to create a pull request if that interests you, but if that's not the direction you want to go, feel free to close/unfix this issue.

