Do major refactor of format code

This gets rid of a lot of duplicated logic that was previously
copy&pasted. This commit alos:
- Makes it easier to implement new formats
- Gets rid of a few bugs
- Makes the system more flexible (more options)
- Adds a lot of additional tests

src/test_utils/example2.rs

@@ -0,0 +1,44 @@
+use std::path::PathBuf;
+
+use crate::Config;
+use crate as confique;
+
+#[derive(Debug, Config)]
+/// A sample configuration for our app.
+pub struct Conf {
+    #[config(nested)]
+    pub http: Http,
+}
+
+/// Configuring the HTTP server of our app.
+#[derive(Debug, Config)]
+pub struct Http {
+    #[config(nested)]
+    pub headers: Headers,
+
+    #[config(nested)]
+    pub log: LogConfig,
+}
+
+#[derive(Debug, Config)]
+pub struct Headers {
+    /// The header in which the reverse proxy specifies the username.
+    #[config(default = "x-username")]
+    pub username: String,
+
+    /// The header in which the reverse proxy specifies the display name.
+    #[config(default = "x-display-name")]
+    pub display_name: String,
+}
+
+
+#[derive(Debug, Config)]
+pub struct LogConfig {
+    /// If set to `true`, the app will log to stdout.
+    #[config(default = true)]
+    pub stdout: bool,
+
+    /// If this is set, the app will write logs to the given file. Of course,
+    /// the app has to have write access to that file.
+    pub file: Option<PathBuf>,
+}