permit local installations
hi,

i'm trying to locally install create-graphql via yarn but i receive this error:

```
npm ERR! argv "/usr/local/bin/node" "/usr/local/bin/npm" "i" "-g" "generator-graphql"
npm ERR! node v7.9.0
npm ERR! npm  v4.2.0
npm ERR! path /usr/local/lib/node_modules
npm ERR! code EACCES
npm ERR! errno -13
npm ERR! syscall access

npm ERR! Error: EACCES: permission denied, access '/usr/local/lib/node_modules'
npm ERR!  { Error: EACCES: permission denied, access '/usr/local/lib/node_modules'
npm ERR!   errno: -13,
npm ERR!   code: 'EACCES',
npm ERR!   syscall: 'access',
npm ERR!   path: '/usr/local/lib/node_modules' }
npm ERR! 
npm ERR! Please try running this command again as root/Administrator.
```

it would be nice to avoid every reference to global packages in order to install create-graphql globally o locally.