Provide a minimal interface exposing SQLite's enable_load_extension

This behavior is untested since we would need to add some extensions in
order to test it.

Fixes #1867

diesel/src/sqlite/connection/mod.rs

@@ -167,6 +167,16 @@ impl SqliteConnection {
         self.transaction_sql(f, "BEGIN EXCLUSIVE")
     }
 
+    /// Enables the `load_extension` SQL function.
+    ///
+    /// See the documentation for the [`enable_load_extension` C function] for
+    /// details.
+    ///
+    /// [`enable_load_extension` C function]: https://www.sqlite.org/c3ref/enable_load_extension.html
+    pub fn enable_load_extension(&self) {
+        self.raw_connection.enable_load_extension();
+    }
+
     fn transaction_sql<T, E, F>(&self, f: F, sql: &str) -> Result<T, E>
     where
         F: FnOnce() -> Result<T, E>,

diesel/src/sqlite/connection/raw.rs

@@ -105,6 +105,10 @@ impl RawConnection {
             ))
         }
     }
+
+    pub fn enable_load_extension(&self) {
+        unsafe { ffi::enable_load_extension(self.internal_connection.as_ptr(), true) }
+    }
 }
 
 impl Drop for RawConnection {