nsqd: config file support

[mreiferson]

consider first-class support for providing flags and options via config files.

I favor the format & behavior used by git, which allows for subsections dedicated to different components (and referencing those via dot notation.)

e.g.:

[core]
    whitespace = blank-at-eol,trailing-space,space-before-tab,blank-at-eof,tabwidth=4
    tabwidth = 4

core.whitespace=blank-at-eol,trailing-space,space-before-tab,blank-at-eof,tabwidth=4
core.tabwidth=4

[mreiferson]

dare I say https://github.com/mojombo/toml?

[mreiferson]

https://github.com/BurntSushi/toml seems to be the cleanest and most robust Go implementation

[mreiferson]

RFR @jehiah

[mreiferson]

according to travis it looks like https://github.com/BurntSushi/toml only supports go1.2 so I'll have to find a more fully compatible package...

[mreiferson]

never mind, got it all working on all versions of Go... had to make some changes to how we handle travis + test.sh (see last commit)

cc @jehiah

[jehiah]

looks good overall.. some questions/thoughts

• should the nsqd.cfg end up in the distribution (and contrib/nsq.spec) as nsqd.cfg or nsqd.cfg.example ?
• what are the future steps to adding config file support for nsqlookupd/nsqadmin? Did we want to do that here as well? (that's needed to make it easy to run those under docker too right?)
• should we leave all options in the nsqd.cfg commented out so that as defaults evolve in the future the binary defaults are used, not defaults from an outdated config file. (thinking aloud)

[mreiferson]

1. yes, it should and it should probably be located and named contrib/nsqd.cfg.example
2. I can do that here, I guess 😁
3. hmmm, I don't think so because you would need to explicitly enable use of a config file via --config. Also, as implemented, command line flags supersede config file options.

[mreiferson]

assuming travis is happy this is RFR @jehiah

[jehiah]

💯 this is some of the worlds cleanest config handling.

[mreiferson]

yea, I think one day I'll pull the guts (the option resolution) out into a separate package... I think it's generally useful