User-defined mapping: add pyproject.toml section parser

[mknorps]

Following @jherland's comment in #260, a user should be able to define a custom mapping in pyproject.toml:

[tool.fawltydeps.custom_mapping]
apache-airflow = ["airflow"]
atlassian-python-api = ["atlassian"]
attrs = ["attr", "attrs"]

[mknorps]

Comment in #287 by @jherland to be considered when implementing:

Also, in relation to what we discussed earlier about using a separate file vs. using a [tool.fawltydeps.custom_mapping] section in pyproject.toml (and in a future PR):

• Can we have this option default to the same file as --config-file, and:
• When it's the same as --config-file, look for the mappings inside the [tool.fawltydeps.custom_mapping] section, or:
• Otherwise, look for the mappings at the top-level of the given TOML file.

[mknorps]

To be able to better distinguish between mapping collected from file and user-defined mapping manually added to pyproject.toml, I propose, we split it to two sections:

[tool.fawltydeps.custom_mapping]
apache-airflow = ["airflow"]
atlassian-python-api = ["atlassian"]
attrs = ["attr", "attrs"]

[tool.fawltydeps.custom_mapping_file]
relative_path/to_my.file.toml