Enable to hijack with arbitrary fields.

[gragragrao]

Why changed

I was reading documentation and realized that I could not hijack with the field that was not username but resistered as USERNAME_FIELD .
In addition to this, I think that it is convenient if I can hijack with any field that can identify the user instance.

Impact range by this PR

We can hijack with any field that can identify the user instance.

Further Develop

If needed, email field can be incorporated into my algorithm.

Comment

This is my first PR to OSS.
Thank you for any help or comment.

[coveralls]

Coverage decreased (-1.7%) to 90.283% when pulling 36e0345 on gragragrao:master into d9b3dc2 on arteria:master.

[coveralls]

Coverage decreased (-0.5%) to 91.429% when pulling 7da338e on gragragrao:master into d9b3dc2 on arteria:master.

[coveralls]

Coverage increased (+0.3%) to 92.245% when pulling 94eb0ea on gragragrao:master into d9b3dc2 on arteria:master.

[walterrenner]

@gragragrao Please update the docs as well. Thanks

[codingjoe]

Hi @gragragrao,

Thank you for your contribution, it's much appreciated. Sadly, it's been a while, but we have new maintainers now helping to get the ball moving again. In fact, the issue you are tackling is one of the first issues we are fixing. I will close this pull-request favoring #216
If you have any objections, suggestions or concerns, please don't be shy raising them. I would also invite you to test and review #216, to evaluate if all your needs have been met.

Best,
Joe