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
+}

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)
+}

internal/integrations/npm_test.go

@@ -0,0 +1,62 @@
+package integrations
+
+import (
+	"github.com/Masterminds/semver"
+	"github.com/Nightapes/go-semantic-release/internal/shared"
+	"github.com/Nightapes/go-semantic-release/pkg/config"
+	"github.com/stretchr/testify/assert"
+	"io/ioutil"
+	"os"
+	"testing"
+)
+
+func TestIntegrations_updateNPM(t *testing.T) {
+	file, err := ioutil.TempFile("", "package")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.Remove(file.Name())
+
+	err = ioutil.WriteFile(file.Name(), []byte(`{
+"name": "test",
+"version": "0.0.0",
+"license": "MIT",
+"scripts": {
+  "ng": "ng",
+  "nx": "nx"
+  }
+}`), 0777)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	testVersion, err := semver.NewVersion("1.2.0")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	i := New(&config.Integrations{NPM: config.IntegrationNPM{
+		Enabled: true,
+		Path:    file.Name(),
+	}}, &shared.ReleaseVersion{
+		Next: shared.ReleaseVersionEntry{
+			Version: testVersion,
+		},
+	})
+
+	assert.NoError(t, i.updateNPM())
+	updatedFile, err := ioutil.ReadFile(file.Name())
+	if err != nil {
+		t.Fatal(err)
+	}
+	assert.Equal(t, `{
+"name": "test",
+"version": "1.2.0",
+"license": "MIT",
+"scripts": {
+  "ng": "ng",
+  "nx": "nx"
+  }
+}`, string(updatedFile))
+
+}