feat: default option for depth from config file

- If `--depth` flag is not defined (or it has an invalid value), a value
  from the config file will be used.
- If no `depth` entry in the config file (or there is no config file),
  the default `usize::MAX` will be used.

Added test cases:
- no config and no flag defined              -> `usize::MAX` should be used
- config defined, but flag is not defined    -> config value should be used
- config is not defined, but flag is defined -> flag value should be used
- both config and flag is defined            -> flag value should be used

Additional changes:

- Fixed some clippy issues.
- Added comments to the example `config.toml` file.
  (copy from flag description)

Closes: #314

Signed-off-by: Balazs Nadasdi <efertone@pm.me>

src/display.rs

@@ -428,7 +428,7 @@ mod tests {
             iso: false,
         };
         DisplayData {
-            initial: initial,
+            initial,
             num_chars_needed_on_left_most: 5,
             base_size: 2_u64.pow(12), // 4.0K
             longest_string_length,