Revert "Only enable --mouse when running from tmux, see #904"

This reverts commit 5f6e310152.

src/output.rs

@@ -100,17 +100,13 @@ impl OutputType {
 
                         // Passing '--mouse' allows mouse scrolling in terminals which do not
                         // support "fake scrolling", see https://github.com/sharkdp/bat/issues/904
-                        // The '--mouse' argument is only supported in less 551 or higher. We do
+                        // The '--mouse' argument is only supported in less 551 or higher.
-                        // not enable this option everywhere because it prevents users from
-                        // drag-selecting text without pressing shift.
-                        if env::var_os("TMUX").is_some() {
                         match less_version {
                             Some(version) if version >= 551 => {
                                 p.arg("--mouse");
                             }
                             _ => {}
                         }
-                        }
                     } else {
                         p.args(args);
                     }