Allows a user to specify an xcode scheme name.

Summary:When running `react-native run-ios`, this feature allows a user to specify which scheme to run.

My project's scheme name is not the same as the xcode project name. Running `react-native run-ios` would error with the following:

```
± |master ↓95 ✓| → react-native run-ios
Found Xcode workspace Poot.xcworkspace
Launching iPhone 6 (9.2)...
Building using "xcodebuild -workspace Poot.xcworkspace -scheme Poot -destination id=2B3E8AAC-DD61-414C-95BD-F4829A8F7CE6 -derivedDataPath build"
User defaults from command line:
    IDEDerivedDataPathOverride = /Users/mrickert/Documents/project/ios/build

xcodebuild: error: The workspace named "Poot" does not contain a scheme named "Poot". The "-list" option can be used to find the names of the schemes in the workspace.
Installing build/Build/Products/Debug-iphonesimulator/Poot.app
An error was encountered processing the command (domain=NSPOSIXErrorDomain, code=2):
Failed to install the requested application
An application bundle was n
Closes https://github.com/facebook/react-native/pull/6157

Differential Revision: D3020313

Pulled By: frantic

fb-gh-sync-id: 9d99fa382b3f50045759acea18eff418b15d511d
shipit-source-id: 9d99fa382b3f50045759acea18eff418b15d511d

local-cli/runIOS/runIOS.js

@@ -33,6 +33,11 @@ function _runIOS(argv, config, resolve, reject) {
     type: 'string',
     required: false,
     default: 'iPhone 6',
+  }, {
+    command: 'scheme',
+    description: 'Explicitly set Xcode scheme to use',
+    type: 'string',
+    required: false,
   }], argv);
 
   process.chdir('ios');
@@ -42,6 +47,7 @@ function _runIOS(argv, config, resolve, reject) {
   }
 
   const inferredSchemeName = path.basename(xcodeProject.name, path.extname(xcodeProject.name));
+  const scheme = args.scheme || inferredSchemeName
   console.log(`Found Xcode ${xcodeProject.isWorkspace ? 'workspace' : 'project'} ${xcodeProject.name}`);
 
   const simulators = parseIOSSimulatorsList(
@@ -63,7 +69,7 @@ function _runIOS(argv, config, resolve, reject) {
 
   const xcodebuildArgs = [
     xcodeProject.isWorkspace ? '-workspace' : '-project', xcodeProject.name,
-    '-scheme', inferredSchemeName,
+    '-scheme', scheme,
     '-destination', `id=${selectedSimulator.udid}`,
     '-derivedDataPath', 'build',
   ];