This commit fixes serving of newly added files

This commit handles the issue in serve command which cannot host
new files. It is worth noting that new file content won't appear
until the tags are added.

Fixed #19
Yes, you're right. If the above regex doesn't match, we get error in 
`header, content = self.re_extract_header.match(content).groups()`
and serving fails. 
I thought this fix can solve this issue, but there is still some unobvious behaviour: until a user doesn't specify tags, the content of the post won't be displayed. 

If you have any ideas about how to handle it, please tell me.