Throws an error when bat is being user as pager.

As mentioned on #1334 `bat` should not be used as a value for `pager`,
this change checks both the balue of `bat` provided as a parameter or
as an environment variable.

src/error.rs

@@ -20,6 +20,10 @@ error_chain! {
             description("unknown syntax"),
             display("unknown syntax: '{}'", name)
         }
+        InvalidPagerValueBat {
+            description("invalid value `bat` for pager property"),
+            display("Use of bat as a pager is disallowed in order to avoid infinite recursion problems")
+        }
     }
 }