Remove date from draft file name and front matter and prepend date when promoting to a post

[bitjockey42]

Addresses #24

This removes the date from the front-matter of a draft post and from the draft filename to comply with how drafts are structured in Jekyll 3.x.x.

Upon promotion of a draft to a post, the date will then be prepended to the filename when moved to _posts/, such that the publish date for the post will match with when the draft was promoted to a post.

[nibrahim]

Merged. Thanks!