[clang-format] Add SpaceBeforeColon option

Summary:
When disabled, this option allows removing the space before colon,
making it act more like the semi-colon. When enabled (default), the
current behavior is not affected.

This mostly affects C++11 loop, initializer list, inheritance list and
container literals:

  class Foo: Bar {}
  Foo::Foo(): a(a) {}
  for (auto i: myList) {}
  f({a: 1, b: 2, c: 3});

Reviewers: krasimir, djasper

Reviewed By: djasper

Subscribers: xvallspl, teemperor, karies, cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D32525

llvm-svn: 326426