Support TOML as another settings file format

[PrajwalM2212]

According to https://github.com/toml-lang/toml
TOML aims to be a minimal configuration file format that's easy to read due to obvious semantics. TOML is designed to map unambiguously to a hash table. TOML should be easy to parse into data structures in a wide variety of languages.

TOML has widespread acceptance, documentation and robust parsers.
Also according to https://www.python.org/dev/peps/pep-0518/, TOML is

1. Human-usable (unlike JSON ),
2. Flexible (unlike configparser),
3. Stems from a standard (unlike configparser)
4. Not overly complex (unlike YAML).

Also TOML has been recommended as the configuration file for software packages to specify their build dependencies.
pyproject.toml is being used for this purpose. https://github.com/pypa/pip/blob/master/pyproject.toml

coala could provide TOML as a additional setting format.

1. By following a well known standard like TOML, we would reduce the learning curve involved.
2. When the current system gets complex, TOML will be a better choice than a custom INI.
3. It provides flexibility, consistency, backwards compatibility and stems from a standard all of which are drawbacks of configparser.

This proposal issue is about supporting a new config file like coafile.toml along with .coafile.

coala/coala#4609

[li-boxuan]

Similar to #702

[PrajwalM2212]

Thanks @li-boxuan for pointing that out. I also think the ideas at #702 should be described individually as well.
I would like to point out the differences tough.
In a way it is a improvement over the first project idea of #702 Standardising .coafile syntax for starters using YAML.

1. This chooses TOML as its a better choice comapred to YAML. https://www.python.org/dev/peps/pep-0518/#other-file-formats
2. Also this does not attempt to use toml as the only config file. It will help starters as due to its popularity and reduced learning curve. But it gives more than that like flexibility, consistency, backwards compatibility and standard.

This is based partially on discussions at coala/cEPs#11 and coala/coala#4511. This issue is derived from coala/coala#4609.

[jayvdb]

See https://gitter.im/coala/coala?at=5bfe8cb5f849b222e0b8bad7 , by @underyx , which provides a very good reason for the settings to be in a standardised file format.

[PrajwalM2212]

Thank you @jayvdb . That is one of the most compelling reasons in defence of the project idea.

[RaiVaibhav]

Great idea and thank you @PrajwalM2212 for pinging me for this, I can't mentor on the project (officially) as I have not applied for the mentor but If you need any help you know how to ping me 😛 and I will try my best to assist you asap.

[RaiVaibhav]

@Makman2 @jayvdb Is this possible that I can help @PrajwalM2212 officially ?