Awsmobile Cli Versions Save

updates from two pull requests: #171 #175

• fixed a typo
• --yes flag implementation for init import strategy
• appsync update workflow, checks on existing apiKeys

• check appsync appId before pull appsync to avoid unnecessary error message
• use yarn (if installed) for react and react-native starters in awsmobile start

Add appsync feature

fixed the init bug reported on issue #77

Add exit codes for api call exceptions