Create pack command for manuscript submission

The pack command is used for preparing a manuscript for submission.
Right now it will

1. Inline inputted latex files
2. Remove comments
3. Copy files to a build directory, including required figures and
update the figure's path in \includegraphics if the figure is moved to
the root directory.

Note that this command can only discover figures included with
\includegraphics{}. More regular expressions can be added for other
environments.