Add barebones but working implementation of model preload (#209, #235)

Add barebones but working implementation of model preload

* add config test for Preload hook
* improve TestProxyManager_StartupHooks
* docs for new hook configuration
* add a .dev to .gitignore

proxy/config_posix_test.go

@@ -100,6 +100,9 @@ func TestConfig_LoadPosix(t *testing.T) {
 	content := `
 macros:
   svr-path: "path/to/server"
+hooks:
+  on_startup:
+    preload: ["model1", "model2"]
 models:
   model1:
     cmd: path/to/cmd --arg1 one
@@ -163,6 +166,11 @@ groups:
 		Macros: map[string]string{
 			"svr-path": "path/to/server",
 		},
+		Hooks: HooksConfig{
+			OnStartup: HookOnStartup{
+				Preload: []string{"model1", "model2"},
+			},
+		},
 		Models: map[string]ModelConfig{
 			"model1": {
 				Cmd:           "path/to/cmd --arg1 one",