Add methods for creating a PUT and DELETE request

Currently the `Request` struct only has methods for GET and POST. I think it would make sense to add also the PUT and DELETE. Up to version `0.5.0`, I generated them by hand like for example:
```rust
let req = ehttp::Request {
method: "PUT".to_string(),
url: put_url,,
body: json,
headers: ehttp::Headers::new(&[("Content-Type", "application/json")]),
};
```
In the version `0.6.0` I see two new fields `mode` and `timeout` were added. however the `ehttp::type` module is private and the `Request` struct can't be constructed outside of the `ehttp` crate.