Add support for #[required] fields

[hnefatl]

Hey! Love the library, I use it a lot for embedding configuration and credentials into embedded devices.

The failing-config example I've added basically describes the use-case that motivated this: I wanted a config that requires some fields (e.g. wifi_ssid) to be set but doesn't necessarily need other fields (e.g. wifi_passkey, ...). I could provide defaults for everything then check for non-default values at runtime, but that's verbose and negates the awesomeness of this crate: being able to control the config at compile-time.

#[toml_cfg::toml_config]
pub struct Config {
    #[required]  // this is new
    wifi_ssid: &'static str,
    #[default("")]
    wifi_passkey: &'static str,
}

[failing-config]
# Oops, I forgot to set `wifi_ssid`.
wifi_passkey = "my_password"

$ cd ../failing-config
$ cargo build --quiet
error: custom attribute panicked
 --> src/lib.rs:3:1
  |
3 | #[toml_cfg::toml_config]
  | ^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: message: Field `wifi_ssid`: required but no value was provided in the config file.

error: could not compile `failing-config` (lib) due to 1 previous error

This is backwards compatible except for error messages: #[default(...)] behaves identically, users have to add #[required] to see any new behaviour.

First two commits are just housekeeping, 3rd commit is implementation+documentation.