Don't load user's configuration

It's possible that users will have configuration settings which break
or slow down gift's operation.  I can't think of a good reason to load
their config file, so we won't.
Include README in pack archive

Pack infrastructure now recognizes a README file in the archive's
root.  That file is rendered as PlDoc and shown on the pack website.
Error message when invoked outside a pack directory

Some gift commands require that the user's present directory be a
pack's root directory.  The old code silently failed back to the help
command in this circumstance.  Now we generate an informative message
instructing the user how to resolve the problem.

I'm slightly hesitant about halting the program from
inside_a_pack_directory.  Throwing an exception might be a better
approach.  It's easy enough to change later if I'm wrong.
Faster 'gift help' response time

In my ongoing quest to make 'gift help' (and other commands) more
responsive, I'm delaying loading some modules until they're really
needed.  library(doc_files) is the biggest offender since it takes
almost 1 full second to load.

This module would probably have autoloaded just fine, but I like
making module dependencies explicit.