Add base config files for common tools.

Includes shell (zsh), editor (emacs), git, and email.

The email configuration has a fair number of moving parts:
- offlineimap for retrieval
- notmuch for indexing
- mutt for reading
- msmtp for sending
Document my e-mail reading and processing setup.

This is so I remember how to duplicate it later since
there's a whole gaggle of interacting packages.