pyproject-toml functionality

[DanNBullock]

Extend functionality to check for and work with pyproject-toml format.

[DanNBullock]

Resources:

• pep518
• poetry pyproject.toml documentation
• pip pyproject.toml documentation

Issues:

• No in-built python capability for machine parsing .toml files? Packages are available, but I don't want to add more dependencies to this. The pep518 documentation seems to imply that it's easily parsable, but I don't see the specific implementation of that.
• The dependency field seems a bit hard to manually parse. That being said, if it were machine-parsable it would be quite useful. Interesting that it seems to also meld a few other CiteAs-relevant formats in there as well.