use keytar package for storing the API key

[philschatz]

This opens a dialog prompting the user to enter an API token when the rate limit is reached or when a repository cannot be found (probably private).

It then stores the API key in the OS keychain/keyring (using atom/node-keytar ) instead of Atom's config.cson which is included when reporting errors.

fixes #20

Screenshot

TODO

• [ ] Improve the wording and UI in the dialog
• [ ] remove the now-extraneous error dialogs
• [ ] decide on a keystore service and account name
• [ ] Maybe provide an automatic migration