Support use of `-e` flag.

See: https://github.com/sindresorhus/ava/pull/815

Specifically: https://github.com/sindresorhus/ava/pull/815#issuecomment-218839461

This is because that PR makes use of the `-e` flag to fork a child process. Use of the `-e` flag causes `spawn-wrap` to disable wrapping that particular process.

It seems entirely possible that `spawn-wrap` could support the `-e` flag when detected by using the `vm` module, or maybe just simply by using `eval`.