Add warning when TOML is not available

[darkdragon-001]

Instead of just having a warning in the comments

python-frontmatter/frontmatter/default_handlers.py

Lines 15 to 16 in 3910ac7

	**Note**: The TOML handler is only available if the `toml <https://pypi.org/project/toml/>`_
	library is installed.

Maybe print a warning to the console instead when TOML is not available?

[eyeseast]

That's a good idea.

I need to decide whether to just include toml by default now that it's (partially) in the standard library.

[merwok]

What about defining packaging extras for the supported formats?

[nikhilweee]

+1 for extra. pip install python-formatter[toml] would be great.