add search command (#33)

* add search command

rfd/search.py

@@ -0,0 +1,13 @@
+def search_threads(threads, keyword=None):
+    """Match deal title and dealer names with keyword specified."""
+
+    if keyword is None:
+        return
+
+    keyword = str(keyword)
+
+    for deal in threads:
+        if keyword.lower() in deal.title.lower() or (
+            deal.dealer_name and keyword.lower() in deal.dealer_name.lower()
+        ):
+            yield deal