Join env var options with "=" instead of " "

Joining them with a space was causing certain styles (e.g. `-grid`) to
be misinterpreted as a separate option.

src/bin/bat/config.rs

@@ -146,8 +146,11 @@ pub fn get_args_from_env_vars() -> Vec<OsString> {
         ("--style", "BAT_STYLE"),
     ]
     .iter()
-    .filter_map(|(flag, key)| env::var(key).ok().map(|var| [flag.to_string(), var]))
-    .flatten()
+    .filter_map(|(flag, key)| {
+        env::var(key)
+            .ok()
+            .map(|var| [flag.to_string(), var].join("="))
+    })
     .map(|a| a.into())
     .collect()
 }