Preserve title metadata in notebooks

[bluss]

Could the default metadata preserve titles? I don't see a reason why it should not be preserved, if the user has set a title.

The following could be the new default. The title matters when producing for example an HTML report.

notebook_metadata_filter="kernelspec,jupytext,title"

[bluss]

See https://github.com/jupyter/nbconvert/pull/672 which says it uses metadata.title if it exists.