Add support for pip-compile's --resolver flag

[tpvasconcelos]

Happy to help with a contribution. But wanted to check first if this is something you would be interested in.

$ python -m piptools compile --help
...
  --resolver [legacy|backtracking]
                                  Choose the dependency resolver.
...

Reference: jazzband/pip-tools#1539

[peterdemin]

Looks good! The PR is welcome!

[peterdemin]

@tpvasconcelos do you still want to contribute this feature? Can I help?

[peterdemin]

I'm gonna take it over.

[peterdemin]

Added in #360

[tpvasconcelos]

Hey @peterdemin sorry I forgot to get back to this one! Thanks for taking a look. I just tested it locally and it works as intended 🚀