…re init`

Summary: This moves us towards our goal of making Pysa trivial to pick up - new projects that run `pyre init` will get the taint models filled out for them, so that you should be able to run `pyre analyze` off the bat. Should have no effect for non-security users.

Reviewed By: grievejia

Differential Revision: D19156586

fbshipit-source-id: 88f4c8e3ae703d9ad0a3cb632fc60d2e94a762d6