clang-format: Enable formatting of protocol buffer definitions.

Almost by accident, clang-format seems to be able to format protocol
buffer definitions (https://code.google.com/p/protobuf/).

The only change is that a space is required between numeric constants
and opening square brackets (for default values). While this might in
theory be used for array subscripts (int val = 4[MyArray]), I have not
seen this pattern in practice much. If this is wrong, we can make this
smarter in the future.