implemented braille support, including --threshold flag

src/cli.rs

@@ -30,6 +30,11 @@ pub struct Cli {
     /// Takes priority over '--width'
     #[arg(short = 'f', long)]
     pub full: bool,
+    /// use a custom threshold when converting to braille
+    ///
+    /// default is 128
+    #[arg(default_value_t = 128, long)]
+    pub threshold: u8,
     /// use a custom character map for output
     ///
     /// The character map is interpreted from dark(first character) to light(last character).