Support proxying chat images via camo

Camo: https://github.com/atmos/camo.  This has a couple advantages over
just allowing images to be dumped as-is:

  - Prevents mixed-content warnings by allowing the server to proxy HTTP
    images to an HTTPS camo instance
  - Protects users' privacy by not exposing their browser directly to
    the image host
  - Allows the camo proxy to intercept and reject bad image sources
    (URLs that are not actually images, gigapixel-sized images likely to
    DoS users' browsers, etc.)

Whitelisting specific domains is supported for cases where the source is
known to be trustworthy.

test/configuration/camoconfig.js

@@ -0,0 +1,25 @@
+const assert = require('assert');
+const CamoConfig = require('../../lib/configuration/camoconfig').CamoConfig;
+
+describe('CamoConfig', () => {
+    describe('#constructor', () => {
+        it('strips trailing slashes from the server', () => {
+            const config = new CamoConfig({
+                camo: {
+                    server: 'http://abc.xyz/'
+                }
+            });
+            assert.strictEqual(config.getServer(), 'http://abc.xyz');
+        });
+
+        it('defaults to enabled=false', () => {
+            assert.strictEqual(new CamoConfig().isEnabled(), false);
+        });
+    });
+
+    describe('#getWhitelistedDomains', () => {
+        it('defaults to an empty array', () => {
+            assert.deepStrictEqual(new CamoConfig().getWhitelistedDomains(), []);
+        });
+    });
+});