Add cmd_stop configuration to better support docker (#35)

Add `cmd_stop` to model configuration to run a command instead of sending a SIGTERM to shutdown a process before swapping.

proxy/config.go

@@ -11,6 +11,7 @@ import (
 
 type ModelConfig struct {
 	Cmd           string   `yaml:"cmd"`
+	CmdStop       string   `yaml:"cmd_stop"`
 	Proxy         string   `yaml:"proxy"`
 	Aliases       []string `yaml:"aliases"`
 	Env           []string `yaml:"env"`
@@ -22,6 +23,9 @@ type ModelConfig struct {
 func (m *ModelConfig) SanitizedCommand() ([]string, error) {
 	return SanitizeCommand(m.Cmd)
 }
+func (m *ModelConfig) SanitizeCommandStop() ([]string, error) {
+	return SanitizeCommand(m.CmdStop)
+}
 
 type Config struct {
 	HealthCheckTimeout int                    `yaml:"healthCheckTimeout"`