Show line numbers on plaintext too

src/snippet.rs

 
 fn format_snippet(filetype: Option<String>, body: String) -> String {
     filetype.map_or(
-        format!("<pre class='plaintext'>{}</pre>", body),
+        format!("<pre class='plaintext'>\n{}\n</pre>", body),
         |filetype| {
             let ps = SyntaxSet::load_defaults_newlines();
             let ts = ThemeSet::load_defaults();
             ps.find_syntax_by_extension(&filetype).map_or(
-                format!("<pre class='plaintext'>{}</pre>", body),
+                format!("<pre class='plaintext'>\n{}\n</pre>", body),
                 |syntax| {
                     highlighted_html_for_string(
                         &htmlescape::decode_html(&body).expect("Invalid HTML"),