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>

config/config.toml

@@ -3,11 +3,29 @@
 #   ~/.config/dust/config.toml
 #   ~/.dust.toml
 
+# Print tree upside down (biggest highest)
 reverse=true
+
+# Subdirectories will not have their path shortened
 display-full-paths=true
+
+# Use file length instead of blocks
 display-apparent-size=true
+
+# No colors will be printed
 no-colors=true
+
+# No percent bars or percentages will be displayed
 no-bars=true
+
+# No total row will be displayed
 skip-total=true
+
+# Do not display hidden files
 ignore-hidden=true
+
+# print sizes in powers of 1000 (e.g., 1.1G)
 iso=true
+
+# Depth to show
+depth=1