Add cache_path: option to RemoteInput.open

[tikkss]

GitHub: GH-5

cache_path: overrides the cache location computed from the URL. The
first entry is the cache ID and the rest is path in the cache:

RemoteInput.open("https://example.com/sample/data.csv",
                 cache_path: "example/sub-directory/data.csv") do |input|
  # Cached at:
  # <system cache directory>/red-remote-input/example/sub-directory/data.csv
  # "example" is cache ID and "sub-directory/data.csv" is path in the cache
end

cache_path must be <cache ID>/<path in cache>. The following paths
are raised with ArgumentError:

• An absolute path
• A path that includes . or ..
• A path that doesn't have path in the cache such as data.csv

cache_path is used as is. Characters aren't replaced unlike the cache
ID computed from the URL.

Extract the cache location logic into RemoteInput::CacheEntry in
preparation. For internal use only.

I'll open a separated pull request not to change the real system cache
in tests.