feat(search): Faceted search based on mod host (curse/modrinth) (#48)

This also adds a commandline argument library (gumdrop) for dealing
with indices - reseting, reconfiguring, and skipping them. I don't
know which library is best for this case, but gumdrop has shorter
compile times and many fewer dependencies than clap, which is why
I chose it.

src/scheduler.rs

@@ -13,9 +13,9 @@ impl Scheduler {
         }
     }
 
-    pub fn run<F, R>(&mut self, interval: std::time::Duration, task: F)
+    pub fn run<F, R>(&mut self, interval: std::time::Duration, mut task: F)
     where
-        F: Fn() -> R + Send + 'static,
+        F: FnMut() -> R + Send + 'static,
         R: std::future::Future<Output = ()> + Send + 'static,
     {
         let future = time::interval(interval).for_each_concurrent(2, move |_| task());