Added HOCON as a config file format

[mockersf]

Default features of the hocon library are disabled.

The two features disabled are:

• serde support, which this crate already does
• include url(...) hocon syntax support, which needs a dependency on reqwest even though it won't be used in most cases

[mehcode]

🙇‍♂️ Thank you for your contribution. I apologize for the late response. Moving forward I want to re-think the design of config as a whole. See #111 for more details.

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Going to close because how we add file formats is going to change pretty drastically. There is a lot of duplicated code in the different formats.

Thank you again though.

As a future point we can probably support a feature flag like hocon_url to turn on include support.