Enable DC specification for in browser use

[GodBleak]

Currently, when attempting to specify a DC as one would when using gramjs in node, it produces this error Chromium:

WebSocket connection to 'wss://149.154.175.10/apiws' failed

Firefox:

Firefox can’t establish a connection to the server at wss://149.154.175.10/apiws.

For development and parity sake, while using gramjs in the browser, one should be able to specify the DC to connect to. This is especially essential for connecting to test DCs to avoid having to modify the source code(PromisedWebSockets.ts:69)

related: #159, #169