Make image proxy configurable

Adds IMAGE_PROXY configuration setting to change image proxy filter behaviour:

- none = No proxy
- http-only = Proxy only non-HTTPS images (default)
- all = Proxy everything

config/config.go

@@ -26,6 +26,7 @@ const (
 	defaultCertDomain       = ""
 	defaultCertCache        = "/tmp/cert_cache"
 	defaultCleanupFrequency = 24
+	defaultProxyImages      = "http-only"
 )
 
 // Config manages configuration parameters.
@@ -217,6 +218,11 @@ func (c *Config) PocketConsumerKey(defaultValue string) string {
 	return c.get("POCKET_CONSUMER_KEY", defaultValue)
 }
 
+// ProxyImages returns "none" to never proxy, "http-only" to proxy non-HTTPS, "all" to always proxy.
+func (c *Config) ProxyImages() string {
+	return c.get("PROXY_IMAGES", defaultProxyImages)
+}
+
 // NewConfig returns a new Config.
 func NewConfig() *Config {
 	cfg := &Config{