Prevent tsd from overwriting fields unrelated to tsd in the config file

tsd config file is currently defined to only handle fields `'path', 'version', 'repo', 'ref', 'bundle', 'stats', 'installed'` in tsd.json. This change allows tsd to support custom data in the config file without erasing it. Tsd still keeps ownership of above mentioned fields.
Related to issue #98