Allow BackoffStrategy to be set via flag, document BackoffStrategy.SetConfig

[twmb]

• Documents that go-nsq calls SetConfig on BackoffStrategy, allowing
  implementors to use config values in their own Calculate implementation.
• Allows full_jitter and exponential strategies to be set via the
  backoff_strategy flag.

[mreiferson]

at a quick glance this looks great - curious to hear your thoughts on my response to your question in #125

[twmb]

I just saw it. I don't mind much either way. If a person wanted to use the config they could just

cfg := nsq.NewConfig()
myStrat.cfg = cfg
cfg.Set("backoff_strategy", myStrat)

Because it's so easy to use the config regardless, it doesn't make much sense to expose setConfig and document it, especially because it also clutters the api.

[mreiferson]

Agreed - let's remove those changes then, thanks!

[mreiferson]

LGTM