Fix plugins prompts on postinstall (#2949)

* Fix plugins prompts on postinstall

In case some plugins (like `nativescript-plugin-firebase`) try to prompt the user on postinstall, users are unable to answer the question when `tns plugin add <name>` is used.
The reason is that the stdin of current process is not passed to the stdin of the `npm install` process and the user has no way to input the result.
Fix this by using "inherit" of current stdio when the console is interactive.

* Update CHANGELOG for 3.1.2

Author: rosen-vladimirov

CHANGELOG.md

@@ -1,5 +1,12 @@
 NativeScript CLI Changelog
 ================
+
+3.1.2 (2017, July 06)
+==
+
+### Fixed
+* [Fixed #2950](https://github.com/NativeScript/nativescript-cli/issues/2950): Unable to provide user input on postinstall of plugin
+
 3.1.1 (2017, June 28)
 ==
 

lib/node-package-manager.ts

@@ -1,5 +1,6 @@
 import * as path from "path";
 import { exported } from "./common/decorators";
+import { isInteractive } from "./common/helpers";
 
 export class NodePackageManager implements INodePackageManager {
 	private static SCOPED_DEPENDENCY_REGEXP = /^(@.+?)(?:@(.+?))?$/;
@@ -194,16 +195,20 @@ export class NodePackageManager implements INodePackageManager {
 	private async getNpmInstallResult(params: string[], cwd: string): Promise<ISpawnResult> {
 		return new Promise<ISpawnResult>((resolve, reject) => {
 			const npmExecutable = this.getNpmExecutableName();
-			let childProcess = this.$childProcess.spawn(npmExecutable, params, { cwd, stdio: "pipe" });
+			const stdioValue = isInteractive() ? "inherit" : "pipe";
+
+			const childProcess = this.$childProcess.spawn(npmExecutable, params, { cwd, stdio: stdioValue });
 
 			let isFulfilled = false;
 			let capturedOut = "";
 			let capturedErr = "";
 
-			childProcess.stdout.on("data", (data: string) => {
-				this.$logger.write(data.toString());
-				capturedOut += data;
-			});
+			if (childProcess.stdout) {
+				childProcess.stdout.on("data", (data: string) => {
+					this.$logger.write(data.toString());
+					capturedOut += data;
+				});
+			}
 
 			if (childProcess.stderr) {
 				childProcess.stderr.on("data", (data: string) => {