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
I don't like `document_obj` name. `document` is an abstract name, of course, but it related to the document being. I mean, when I see the `document` variable, I think that's an instance of some class rather then path.

So I'd prefer to rename:

* `document` -> `document_path`
* `document_obj` -> `document`