Generate index.json with pre commit hook

[nohponex]

Use pre commit hook to generate the index.json

Use CI #2 to validate index.json structure

[nohponex]

@terietor, @alkallio What would be the best approach to generate the index.json file from available files in data_template ?

1. pre commit hook
2. a commit & push from CI after successful "build"
3. manually

index.json is currently served from https://raw.githubusercontent.com/phramework/schema-data_template/master/data_template/, would it be better if we use gh-pages?