Support passing params to list queries

Adding this option to support passing query params when
listing customers as per:

https://developer.helpscout.com/mailbox-api/endpoints/customers/list/

lib/help_scout/modules/listable.rb

@@ -2,12 +2,13 @@
 
 module HelpScout
   module Listable
-    def list(id: HelpScout.default_mailbox, page: nil)
-      HelpScout.api.get(list_path(id), page: page).embedded_list.map { |e| new e }
+    def list(id: HelpScout.default_mailbox, page: nil, **params)
+      HelpScout.api.get(list_path(id), { page: page }.merge(params)).embedded_list.map { |e| new e }
     end
 
     def list_path(_)
       base_path
     end
   end
 end
+

spec/integration/customer_spec.rb

@@ -8,4 +8,12 @@
 
   include_examples 'getable integration'
   include_examples 'listable integration'
+
+  describe 'list' do
+    it "finds customers filtered with query" do
+      result = described_class.list(query: '(email="[email protected]")')
+      expect(result).to be_a Array
+      expect(result).to all(be_a(described_class))
+    end
+  end
 end