Adding support for reading credentials from a netrc file on *nix systems

[JeffMv]

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Proposed changes

Added support for providing credentials from a netrc file on *nix systems.
The code and process has been based on the code of the coursera-dl/coursera-dl repo (which was working and tested few years back) and adapted to provide that login option for edx-dl users.

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• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

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[coveralls]

Coverage increased (+0.3%) to 47.955% when pulling 8e58984 on JeffMv:master into 265718c on coursera-dl:master.

[de-code]

Seems like a good feature to have.