Add --publish-date-from-bibtex flag

[arichardson]

When set, the publishDate field uses the bibtex value instead of the current time. This is useful to avoid all .md files changing whenever the bibtex file is imported.

Additionally, the bibtex importer now checks whether publishDate is already set in the metadata and does not replace it in that case.

[gcushen]

This is useful to avoid all .md files changing whenever the bibtex file is imported.

The Markdown files should only be overwritten if the "overwrite" flag is present, if that's not currently the case, there's another bug to fix.

Assuming you are using the "overwrite" flag, it might be interesting to investigate similar open source tools and explore storing/caching some metadata regarding what was processed when in order to optimise the tool and prevent things such as refreshing the page publication date.

For example, Hugo creates its own cache and resources folders for somewhat similar purposes.

The current changes you've proposed look like they would cause issues for users though - see the comments I left on your PR code.

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