Allow a custom filter function instead of glob

[callumlocke]

1. I want to be able to pass a custom function that examines filenames and returns true/false
2. I propose renaming the glob option to filter (and keeping glob as an alias, for back compatibility)

let me know if you agree with one or both of the above, and I'll PR!

[amasad]

Would love to pull, but let's make it a filter function in addition to glob, I don't like overloading the same arg with multiple types

[callumlocke]

ok 👍

[stefanpenner]

We are still open for a pull request, but as this as this has gone stale I will close.