[clang-format] Support text proto extensions

Summary:
This adds support for text proto extensions, like:
```
msg {
  [type.type/ext] {
    key: value
  }
}
```

Reviewers: djasper

Reviewed By: djasper

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D43180

llvm-svn: 324995