First production release

Enhancements

• Accept lists of target objects at the end of _targets.R (#253).
• Deprecate tar_pipeline() and tar_bind() because of the above (#253).
• Always show a special message when the pipeline finishes (#258, @petrbouchal).
• Disable visNetwork stabilization (#264, @mattwarkentin).
• Use default visNetwork font size.
• Relay errors as condition messages if error is "continue" (#267, @liutiming).