nsqd: config file support

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
````