ignore git ignored files by default

I had a config like this on one react-native project:
```
documents: ['**/*.gql']
```
this ran like 40 seconds when trying to generate. Why?
Because it is traversing whole node_modules.
So I would like to have 2 new features:
1. ignore git ignored files by default
2. When I have too many files, emit a warning to stdout and recommend to limit the glob expression