Add regex file filtering policy

[kriswuollett]

Refactors the file policy code and adds an implementation for a regex filter. Motivating use case is filtering out test protos that may be in third party repos. Used URI-like prefix to allow for later easier addition of other filter policy implementations.

Closes #168

An example usage of the regex policy that was tested manually:

[protobuf]
url = "github.com/protocolbuffers/protobuf"
protocol = "https"
revision = "938d3bdf860dfc3c9a55bc5961ce1ecdb1a87e0f"
allow_policies = ["google/protobuf/*"]
deny_policies = ["re://compiler|bridge|sample|test"]
content_roots = ["src"]

Yields:

% tree protos/google/protobuf 
protos/google/protobuf
├── any.proto
├── api.proto
├── cpp_features.proto
├── descriptor.proto
├── duration.proto
├── empty.proto
├── field_mask.proto
├── internal_options.proto
├── late_loaded_option.proto
├── late_loaded_option_user.proto
├── source_context.proto
├── struct.proto
├── timestamp.proto
├── type.proto
├── util
│   ├── json_format.proto
│   └── json_format_proto3.proto
└── wrappers.proto

2 directories, 17 files

[rtimush]

Looks great to me, thank you for the contribution!