Fixes #411

This is still sub optimal since a stack trace is shown instead of a nice error
message, but at least adds a human readable context with full file path instead
of a generic 'Unwrap() went wrong".

Before:

```
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Os { code: 2, kind: NotFound, message: "No such file or directory" }', src/libcore/result.rs:1165:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
```

Now:

```
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: "Couldn\'t open config file: cbindgen.toml."', src/libcore/result.rs:1165:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
```

 * Initialize struct literal with list-initializer for C++11 standard (#401)
 * Surround namespace with __cplusplus ifdef in cpp_compat mode
 * Add support for --quiet flag
 * Map char to char32_t
 * Improve binding_crate_ref() error message
 * avoid prematurely returning during expansion
 * Add support for adding "using namespace" statements

 * Improved error message for missing config file. #422
 * Add missing header for char32_t. #414

This is technically a breaking change for C++, unfortunately.

Fixes #423.

 * Made rust char map to uint32_t. #424