Initial implementation of pluggable commands system

The CommandManager class in command.py loads names of all
available commands and invokes the one that was specified
by a user in command line.

Closes #2
Implementation of the serve command

This commit adds implementation of the serve command which is designated
to host generated documents at the local http server. The output path of
of the build command is used as a core directory to start a server.

Closes #3
Merge pull request #6 from ikalnitsky/commands

Pluggable CLI