Consider MIT license

[kylewillmon]

The current license on this repository is GPLv3+, which may be too restrictive for community extensions. We should consider a more permissive license, such as MIT.

[matt-phylum]

We should sort this out or put in a notice that all contributions have copyright assigned to Phylum. Otherwise we may need to track down contributors and get them to relicense their contributions.

[kylewillmon]

Our CLA now covers this repository, so we have the authority to relicense contributions without tracking down contributors (as long as the new license is also a free software license per the text of the CLA).