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>

tests/test_flags.rs

@@ -147,7 +147,7 @@ pub fn test_output_screen_reader() {
     assert!(output.contains("a_file     2"));
 
     // Verify no 'symbols' reported by screen reader
-    assert!(!output.contains("│"));
+    assert!(!output.contains('│'));
 
     for block in ['█', '▓', '▒', '░'] {
         assert!(!output.contains(block));