allow command line arguments to override previous values

this is useful when some default options are set in `HGREP_DEFAULT_OPTS`
and user wants to change them by subsequent arguments in command line

CONTRIBUTING.md

@@ -56,7 +56,7 @@ Once some of them fail, the error messages contain the differences. You need to
 
 ```sh
 # Run snapshot tests
-cargo test
+cargo insta test
 # Review snapshot diffs
 cargo insta review
 # After accepting the diffs, remember to add the changes

src/main.rs

@@ -87,6 +87,7 @@ fn command() -> Command {
             For more details, visit https://github.com/rhysd/hgrep#readme"
         )
         .no_binary_name(true)
+        .args_override_self(true)
         .arg(
             Arg::new("min-context")
                 .short('c')
@@ -916,6 +917,10 @@ mod tests {
                 "--list-themes",
             ]
         );
+        snapshot_test!(
+            override_options,
+            ["--theme", "ayu-dark", "--theme", "OneHalfDark"]
+        );
 
         macro_rules! snapshot_error_test {
             ($name:ident, $args:expr) => {