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_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))
+
+}