Add login token support for multiple bugzillas

The tokens are saved in ~/.bugz_tokens, which is a file that can be read
by configparser.

Each section in this file corresponds to a section in the configuration
files. The token setting is the token passed back to us by Bugzilla's
login command.

This fixes #78.
Migrate old token file into the new system

This migrates the old single token file to the new token configuration
file.

The old token will be assigned to the default connection, or the one you
use with the --connection switch when the old file is detected.

If this is not correct, you will need to edit ~/.bugz_tokens and change
the section name to match the appropriate connection.

This is also related to #78.