add incsearch option to choose if incremental search is enabled

src/main.cc

@@ -155,6 +155,8 @@ void do_search(Context& context)
                         RegisterManager::instance()['/'] = ex;
                     context.push_jump();
                 }
+                else if (not context.options()["incsearch"].as_int())
+                    return;
 
                 context.editor().select(std::bind(select_next_match, _1, ex), mode);
             }

src/option_manager.cc

@@ -99,6 +99,7 @@ GlobalOptions::GlobalOptions()
     set_option("BOM", Option("no"));
     set_option("shell", Option("sh"));
     set_option("complete_prefix", Option(1));
+    set_option("incsearch", Option(1));
     set_option("ignored_files", Option(R"(^(\..*|.*\.(o|so|a))$)"));
 }