Update icons.rs

Add Markdown icon for Julia Markdown documents (same convention as for `*.Rmd`)

src/output/icons.rs

@@ -214,6 +214,7 @@ pub fn icon_for_file(file: &File<'_>) -> char {
             "jfif"          => '\u{f1c5}', // 
             "jif"           => '\u{f1c5}', // 
             "jl"            => '\u{e624}', // 
+            "jmd"           => '\u{f48a}', // 
             "jpe"           => '\u{f1c5}', // 
             "jpeg"          => '\u{f1c5}', // 
             "jpg"           => '\u{f1c5}', // 