s3cache: expose add'l config options as URL params

this should allow using at least some s3-compatible services like minio.

Fixes #120, #147

README.md

@@ -108,12 +108,28 @@ enabled using the `-cache` flag.  It supports the following values:
    cache items no longer than 4 hours.
  - directory on local disk (e.g. `/tmp/imageproxy`) - will cache images
    on disk
+
  - s3 URL (e.g. `s3://region/bucket-name/optional-path-prefix`) - will cache
    images on Amazon S3.  This requires either an IAM role and instance profile
    with access to your your bucket or `AWS_ACCESS_KEY_ID` and `AWS_SECRET_KEY`
    environmental variables be set. (Additional methods of loading credentials
    are documented in the [aws-sdk-go session
    package](https://docs.aws.amazon.com/sdk-for-go/api/aws/session/)).
+
+   Additional configuration options ([further documented here][aws-options])
+   may be specified as URL query string parameters, which are mostly useful
+   when working with s3-compatible services:
+   - "endpoint" - specify an alternate API endpoint
+   - "disableSSL" - set to "1" to disable SSL when calling the API
+   - "s3ForcePathStyle" - set to "1" to force the request to use path-style addressing
+
+   For example, when working with [minio](https://minio.io), which doesn't use
+   regions, provide a dummy region value and custom endpoint value:
+
+       s3://fake-region/bucket/folder?endpoint=minio:9000&disableSSL=1&s3ForcePathStyle=1
+
+   [aws-options]: https://docs.aws.amazon.com/sdk-for-go/api/aws/#Config
+
  - gcs URL (e.g. `gcs://bucket-name/optional-path-prefix`) - will cache images
    on Google Cloud Storage. Authentication is documented in Google's
    [Application Default Credentials