Support specifying which packages and versions to update.

[nnja]

The current behavior for pip-compile --upgrade updates all requirements. This becomes complicated for large .in files when that's not always the desired behavior.

This patch introduces a new flag, --upgrade-package which has the following features:

Upgrade only a specific package to the latest version:

$ pip-compile --upgrade-package flask

Upgrade a specific package to a specific version:

$ pip-compile --upgrade-package flask==0.10.1 

Upgrade multiple packages at a time, by passing in multiple arguments:

$ pip-compile --upgrade-package flask==0.10.1 --upgrade-package werkzeug

Additional output was added when using the --verbose flag to list which packages were updated, and to which version.

[nnja]

Pushed some updates - the build is now green, except for https://travis-ci.org/nvie/pip-tools/jobs/176202798 piptools/utils.py:22:1: E305 expected 2 blank lines after class or function definition, found 1 which seems unrelated to my changes, since I have not modified utils.py

[nvie]

I love this idea, thanks for your contribution, @nnja!

The way pip-compile's default invocation vs the upgrade logic works is quite clever. Basically, what happens is that if an output file exists (and the --upgrade flag is not given), it will add the current pins from the output file into the list of input specs, and then "just" trigger the resolver.

In that light, using this new -P <pkg> flag really means "don't perform the above-mentioned process for this package <pkg>". Or, when a pin is given: "ditto, but also pin it to <pin> while you're at it".

So really all you need to do here is to filter the list of "existing pins" and therefore discarding those explicit pins in favor of the stuff provided on the command line. I've gone ahead and implemented that in d0dd26a, which as you can see does not require any special handling for this feature at all. (It's perhaps best to look at the net-diff against master for the best view of the total change required for this.)

Thanks a lot!