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/integrations.go

@@ -0,0 +1,26 @@
+package integrations
+
+import (
+	"github.com/Nightapes/go-semantic-release/internal/shared"
+	"github.com/Nightapes/go-semantic-release/pkg/config"
+)
+
+// Integrations struct
+type Integrations struct {
+	version *shared.ReleaseVersion
+	config  *config.Integrations
+}
+
+func New(config *config.Integrations, version *shared.ReleaseVersion) *Integrations {
+	return &Integrations{
+		config:  config,
+		version: version,
+	}
+}
+
+func (i Integrations) Run() error {
+	if i.config.NPM.Enabled {
+		return i.updateNPM()
+	}
+	return nil
+}