allow ast command to parse JSON files
allow ast command to parse JSON files

Summary:`flow ast` expected to get a JS file, so failed with syntax errors if you tried to parse a JSON file.

now, files that end with `.json`, or when the `--type json` flag is set, will get parsed as JSON.

Reviewed By: gabelevi

Differential Revision: D3062059

fb-gh-sync-id: 383c14bc7de1bcef2af13307a45db9d77f25629b
shipit-source-id: 383c14bc7de1bcef2af13307a45db9d77f25629b

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