io_poller docs, and features.

[Anotra]

What?

Changes all relate to io_poller
Added documentation, extra param struct io_poller *io to io_poller_callbacks and a new function discord_get_io_poller()

Why?

Eliminate the requirement for the user to have a separate thread when they want to poll their own file descriptors

simple example using stdin

static void
on_stdin(struct io_poller *io, enum io_poller_events events, void *data) {
  struct discord *client = data;
  char buf[4096];
  int n = read(STDIN_FILENO, buf, sizeof buf);
  write(STDOUT_FILENO, buf, n);
}

int
main(void) {
  struct discord *client = discord_config_init("config.json");
  struct io_poller *io = discord_get_io_poller(client);
  io_poller_socket_add(io, STDIN_FILENO, IO_POLLER_IN, on_stdin, client);
  discord_run(client);
}