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
This is a sort of global state, since we assign some value to some class-attribute (not an instance-attribute). 

I'm not sure, but I think we can create an instance of `HolocronHandler`, pass this value to its constructor and use this instance as argument in `HTTPServer`.