gamebanana: add function for downloading mods

kinda janky with the implementation right now, later i would like to
improve the way files are copied and allow for mods with multiple
downloads attached to the mod. probably split off some of the
functionality later to add installing mods not on gamebanana easier.

gamebanana/download_test.go

@@ -0,0 +1,24 @@
+package gamebanana
+
+import (
+	"os"
+	"path"
+	"testing"
+)
+
+func TestDownloadGameBananaMod(t *testing.T) {
+	tmpdir := t.TempDir()
+	err := DownloadGameBananaMod("602180", tmpdir)
+	if err != nil {
+		t.Error("failed to download game banana mod", err)
+	}
+
+	// udate is the time the mod was last updated
+	// in unix time, used here as a version number
+	// modID@udate
+	expectedPathToExist := path.Join(tmpdir, "602180@1750696716")
+
+	if _, err := os.Stat(expectedPathToExist); err != nil {
+		t.Error("mod doesn't have expected name or doesn't exist")
+	}
+}