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/search/indexing/curseforge_import.rs

@@ -2,6 +2,7 @@ use super::IndexingError;
 use crate::search::UploadSearchMod;
 use log::info;
 use serde::{Deserialize, Serialize};
+use std::borrow::Cow;
 
 #[derive(Serialize, Deserialize, Debug)]
 #[serde(rename_all = "camelCase")]
@@ -200,7 +201,8 @@ pub async fn index_curseforge(
             date_modified: modified.to_string(),
             modified_timestamp: modified.timestamp(),
             latest_version,
-            empty: std::borrow::Cow::Borrowed("{}{}{}"),
+            host: Cow::Borrowed("curseforge"),
+            empty: Cow::Borrowed("{}{}{}"),
         })
     }