chore: flexible configuration of environment value transfer (#235)

**Reason:**
I have to use drone-ssh to work with Windows SSH. Initially, drone-ssh is written so that it transmits environment variables through the `export` command. Which makes it unsuitable for working with Power Shell.

**Solution:**
I have added a new option to configure environment variable commands formatting, with default value: `export {NAME}={VALUE}`. When I use drone-ssh with PowerShell I set this option like this: `$env:{NAME} = {VALUE}`.

plugin.go

@@ -40,6 +40,7 @@ type (
 		Sync              bool
 		Ciphers           []string
 		UseInsecureCipher bool
+		EnvsFormat        string
 	}
 
 	// Plugin structure
@@ -103,7 +104,7 @@ func (p Plugin) exec(host string, wg *sync.WaitGroup, errChannel chan error) {
 	for _, key := range p.Config.Envs {
 		key = strings.ToUpper(key)
 		if val, found := os.LookupEnv(key); found {
-			env = append(env, "export "+key+"="+escapeArg(val))
+			env = append(env, p.format(p.Config.EnvsFormat, "{NAME}", key, "{VALUE}", escapeArg(val)))
 		}
 	}
 
@@ -150,6 +151,11 @@ loop:
 	}
 }
 
+func (p Plugin) format(format string, args ...string) string {
+	r := strings.NewReplacer(args...)
+	return r.Replace(format)
+}
+
 func (p Plugin) log(host string, message ...interface{}) {
 	if p.Writer == nil {
 		p.Writer = os.Stdout