add node engine to package.json and call checkNodeVersion function in cli

[conorhastings]

closes #82

cc @gaearon

[vjeux]

Can you comment on how you tested this feature? Thanks!

[conorhastings]

@vjeux sure:

1. looked into how react-native cli uses their checkNodeVersion
2. added the engine to the package and added the call to checkNodeVersion in the run of this cli
3. used nvm to switch to node 0.12.7
4. mved the code from my working branch of the code into the globally installed node version folder
5. edited local code to insert the engine (not ideal, would love feedback on how to do it better)
   
6. ran create-react-app my-app and observed error
   

[vjeux]

Awesome test plan thanks!

It looks like it still lets you keep running the command anyway (it just prints a red warning), should we process.exit(1) there?

[conorhastings]

@vjeux updated

[vjeux]

Thanks!