make new_post error a bit more intelligent

Signed-off-by: Chris “Kwpolska” Warrick <kwpolska@gmail.com>
trailing slash

Signed-off-by: Chris “Kwpolska” Warrick <kwpolska@gmail.com>
update `nikola help` output in the manual

Signed-off-by: Chris “Kwpolska” Warrick <kwpolska@gmail.com>
rewrite a nonsense sentence in the manual re new_post (via @gour)

Signed-off-by: Chris “Kwpolska” Warrick <kwpolska@gmail.com>