Make marking articles as drafts case-insensitive

Previously if you tried to mark an article as a draft by using a different
casing (for example, draft) you would get a warning when building:
`Unknown status Draft for file foo.md, skipping it.` This uses a
case-insensitive comparison when looking at article status instead. I
believe this behavior is a little easier for new Pelican users.
Merge pull request #1179 from jculpon/draft-casing

Make marking articles as drafts case-insensitive