feat(integrations): add first simple npm integration

Integrations are simple helpers to make integration with existing tools easier.
At basic npm support, the integration will set the version before release to the `package.json`

```yml
integrations:
  npm:
    enabled: true
```

internal/integrations/npm.go

@@ -0,0 +1,28 @@
+package integrations
+
+import (
+	log "github.com/sirupsen/logrus"
+	"github.com/tidwall/sjson"
+	"io/ioutil"
+)
+
+func (i *Integrations) updateNPM() error {
+
+	npmConfig := i.config.NPM
+	if npmConfig.Path == "" {
+		npmConfig.Path = "./package.json"
+	}
+
+	log.Debugf("Set version %s to %s", i.version.Next.Version, npmConfig.Path)
+	data, err := ioutil.ReadFile(npmConfig.Path)
+	if err != nil {
+		return err
+	}
+
+	newData, err := sjson.Set(string(data), "version", i.version.Next.Version)
+	if err != nil {
+		return err
+	}
+
+	return ioutil.WriteFile(npmConfig.Path, []byte(newData), 0777)
+}