add external_checksum and checksum_algorithm

[aaschaer]

Adding these as explicit named arguments along with doc for how they are used by Transfer.

First PR since the linting and pytest changes, but I think make test did the right thing?

[sirosen]

This looks good to me (and 👍 for documenting the parameters to add_item), but it looks like black is failing: https://travis-ci.com/globus/globus-sdk-python/jobs/228329637

This is sort of an issue with the way that I setup the tox.ini -- make test doesn't run black checking because I had issues getting tox -e lint to do what I wanted on Travis.
I have thoughts on ways of re-doing things, but it's just not been a priority.

If you run make autoformat it should fix any formatting issues. And I'd just amend with the result, since it's just a formatting fix.

[sirosen]

I gave this a final check before merging and I noticed something suspicious about the spacing of the parameters as listed in the docstring. Passing it through sphinx (make docs) and looking at the resulting html, I can confirm that it doesn't come out quite right (or at least, not the same as our other docs).

We get this:

instead of

All that should be needed to tweak this is a little bit of indenting to make sphinx happy.
I've put the change that I believe makes this work here: 0986398
You can just git cherry-pick 0986398 if you want to pull in that change.

[aaschaer]

chery-picked the commit and the indenting seems fixed, thank you!