This commit allow routes to be declared for methods outside of the
standard HTTP method set. Specifically, it enables declaring routes for
any method in the IANA Method Registry:
```rust
#[route(LINK, uri = "/<foo>")]
fn link() { ... }
#[route("VERSION-CONTROL", uri = "/<foo>")]
fn version_control() { ... }
```
The `Method` type has gained variants for each registered method.
Breaking changes:
- `Method::from_str()` no longer parses mixed-case method names.
- `Method` is marked as non-exhaustive.
- `Method::supports_payload()` removed in favor of
`Method::allows_request_body()`.
Resolves#232.