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 think we need a bit rewrite that docstring, since I don't like our point that a serve command is run inside a directory with `_build`. Actually, the serve command just create a simple http server that share files from `_build` folder. Please, fix it.

BTW, change please `/_build` to `_build`. The leading slash is looks like a root of *nix file systems.