Bug: Cannot configure custom polling_strategy from YAML config

[mensfeld]

Summary

Custom polling strategies cannot be specified in YAML configuration files.

Steps to Reproduce

# shoryuken.yml
polling_strategy: MyCustomPolling
queues:
  - my_queue

ArgumentError: MyCustomPolling is not a valid polling_strategy

Root Cause

The polling_strategy option only accepts class constants, not strings. YAML files provide strings.

Proposed Fix

Add string handling in Options#polling_strategy:

when String
  Object.const_get(strategy)

Additional Bug

The add_group method also lacks a polling_strategy option.

Original Issue

Originally reported in #675, labeled as bug, closed by stale bot.