Add shx as a dependency for testing

[nfischer]

Tests in #524 are breaking because there's no good way to test the cwd option. There are also other parts where we try to use grep, but can't use it if it doesn't exist on Windows. If we import shx as a dev dependency, then we can get away with using shx pwd and shx grep for these tests.

[nfischer]

Also, just realized that shx only supports v4+. I think the best thing is to just expand it with polyfills where necessary to support v0.11+. This might be a bit of work, but it should be achievable.

[nfischer]

shx now has the proper support. We need to fix some of the remaining shx issues and then make a new release. Once ShellJS depends on that release, we can close this.

[nfischer]

shx 0.2.0 has been released, so this is no longer blocked.

[nfischer]

Fixed by #550