Extract Twitter secrets from _config.yml or ENV

In this version the plugin looks first to `_config.yml` for the Twitter secrets. If not present there, it'll look to `ENV` as it did before. This update should not break any implementations of `v1.1.1`, but will allow users to keep their Twitter secrets in the `_config.yml`.

I updated the README doc to describe the new procedures.

I didn't update the Gemspec or any version info.
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