Add --quiet options to build and check

[Julian]

It'd be nice, particularly for pep517.check, to selectively show less noisy output (e.g. simply whether things were successfully built).

[pradyunsg]

PRs welcome. :)

[takluyver]

We've come to an agreement that the functionality to create isolated build environments and install build dependencies will live in the PyPA build project. The pep517 library will continue to exist as the lower-level piece to work with the interfaces defined by PEP 517, but the pep517.envbuild module and the little command-line interfaces which use it (pep517.build, pep517.check and pep517.meta) will gradually be deprecated and eventually removed as build becomes more stable. See #91 for more on that process.

So I'm closing this, as there won't be any further significant work here on these pieces. Hopefully build already meets your needs better.