feat(publish): watch mode

Fixes https://github.com/privatenumber/link/issues/22 This PR adds a `--watch` flag to the publish command. If the source file is deleted and re-created (as is the case for some build systems, such as https://github.com/callstack/react-native-builder-bob), then the `--watch` flag will re-establish the hard link with the new file in the source file path.