Add config file and use shared header/footer

[kabili207]

This contains a few changes:

• Modifies index.php and outgoing.php to use shared top/bottom includes to reduce redundant code
• Moves bearer token and subscription settings to config.php
  • The config.php is added to the .gitignore to prevent accidental commit of sensitive data, as well as to prevent overwriting of user configs
  • An empty config.php.dist file is included as a template
• Fixes to handling of custom fields
  • According to the TrackHive API, these are supposed to be Objects with key/value pairs, not Strings
  • This allows better compatibility with other systems adding/reading from the TrackHive API
  • This mismatch caused ParcelPony to break if custom_fields were missing or in a different format
  • For example, I use a milter on my mail server to automatically add any tracking numbers detected in incoming emails
  • I made sure to check for backwards-compatibility

[fireshaper]

Thanks for helping out on the project! I merged your commits.