Babel transpilation skipped if codemod is installed globally

When a codemod is installed globally (`npm i -g`), for example because a custom CLI command is supplied that wraps `jscodeshift`, Babel transpilation is skipped. This is because `babel-register` will not process any files that have a `node_modules` in their path (see https://github.com/babel/babel/blob/6.x/packages/babel-register/src/node.js#L91). But this is always the case when the package is installed globally (e.g. at `/usr/local/lib/node_modules`).

It would be possible to customize Babel's behavior when a custom `ignore` or `only` option could be used, but unfortunately these options are hardcoded in jscodeshift (related #245).

See also https://github.com/babel/website/issues/703