Add -C as a short version of --config-settings

[FFY00]

Signed-off-by: Filipe Laíns lains@riseup.net

[rgommers]

This is quite useful, thanks @FFY00. I know @henryiii was planning to do this exact same thing as well. This is a UX improvement, and also aligns pip with pypa/build, so +1 from me.

[henryiii]

Just to follow up: Pip calls this --config-settings, while build calls it --config-setting and -C, so this would allow tutorials and such to use -C to work across both build and pip. Also, setting a config setting contains an equal sign, so this transforms --config-settings=build-dir=build into -Cbuild-dir=build, which is much nicer than having dual equals signs, I think. Finally, for PEP 517 backends, this is the core mechanic for communicating to the backend, and may easily be done multiple times in a single call, so having a short flag for it is warranted, I think.

[FFY00]

@pradyunsg could you have a look? This is a very small, but super useful, change. Thanks!