Add $LESSOPEN and $LESSCLOSE support (#2444)

src/config.rs

@@ -90,6 +90,10 @@ pub struct Config<'a> {
     /// Whether or not to allow custom assets. If this is false or if custom assets (a.k.a.
     /// cached assets) are not available, assets from the binary will be used instead.
     pub use_custom_assets: bool,
+
+    // Whether or not to use $LESSOPEN if set
+    #[cfg(feature = "lessopen")]
+    pub use_lessopen: bool,
 }
 
 #[cfg(all(feature = "minimal-application", feature = "paging"))]