Implemented client credential grant type

[prakharjoshi]

Implemented client credential grant type without disturbing the default setting for authenticate code grant type

[prakharjoshi]

related to #138

[mrichar1]

Hi,

Thanks for this PR. However there are a couple of 'tidying' things that would be needed before the code can be fully reviewed:

• Please put the change comments into the commit message, not README.rst

• Please update the documentation to explain this change.

• from authomatic.providers import AuthorizationProvider as authprovider is redundant, given that providers.AuthorizationProvider is used elsewhere.

• Please try to not make whitespace changes, or commented-out code unless they are specific to your PR. e.g. Including core.py seems unnecessary

• There are some places where the code could be more DRY - e.g. oauth2.py lines 130 where the same variable value is set twice.

Hopefully these things can be cleaned up and we can then properly review your request!

[mrichar1]

After several years of inactivity, authomatic is now under community management, and we have just released a new stable version (1.0.0).

We are now reviewing all issues and PRs and hoping to begin work to solve as many of these as possible.

We are keen to find out which issues still apply, and which PRs are still required/are likely to merge cleanly into the current code. We are aiming to review them all, but any help with prioritisation would be very useful!

If you are still interested in having this issue/PR resolved, or are able to help us work on it, please reply to this message. That way we know which issues are most important to the community.