This is a somewhat basic implementation of `-f-` (#7), with unit tests.
Changes include:
1. The internals of the `pattern` function have been refactored to avoid
code duplication, but there's a lot more we could do. Right now we
read the entire pattern list into a `Vec`.
2. There's now a `WorkDir::pipe` command that allows sending standard
input to `rg` when testing.
Not implemented: aho-corasick.
