Add parse extra bindings to builder

**Parse -> extra_bindings** is available under _cbindgen.toml_, but missing in the _Builder_.

I added a pull request:

```rust
#[allow(unused)]
pub fn with_parse_extra_bindings<S: AsRef<str>>(mut self, extra_bindings: &[S]) -> Builder {
    self.config.parse.extra_bindings = extra_bindings
        .iter()
        .map(|x| String::from(x.as_ref()))
        .collect();
    self
}
```