Add --reset-author flag

[sorenlouv]

Closes #146

Add --reset-author flag which makes it possible to change the commit author to yourself.
This is useful when the original commit was made by a bot and the the CLA check needs to be bypassed.

Example usage:

backport --reset-author

@spalger

[spalger]

LGTM, tested with elastic/kibana#44717, but I used the documented --resetAuthor flag, do both kebab and camel case flags work?

[sorenlouv]

I used the documented --resetAuthor flag, do both kebab and camel case flags work?

@spalger Good point. Yes both work due to the way yargs-parser works.

I've now replaced --resetAuthor with --reset-author in the docs for consistency. I've also added a test that verifies this behaviour on the cli level.