Add Configuration support for custom Injector and Extractor

[martinxsliu]

Re: #262

This PR adds the ability to specify custom Injectors and Extractors when creating new Tracers using the Configuration struct.

Usage:

c := jaegerconfig.Configuration{}

tracer, closer, err := c.New(
	"service",
	jaegerconfig.Injector("custom.format", CustomInjector{}),
	jaegerconfig.Extractor("custom.format", CustomExtractor{}),
)

[codecov]

Codecov Report

Merging #263 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #263      +/-   ##
==========================================
+ Coverage   84.95%   85.02%   +0.07%     
==========================================
  Files          51       51              
  Lines        2705     2718      +13     
==========================================
+ Hits         2298     2311      +13     
  Misses        287      287              
  Partials      120      120

Impacted Files	Coverage Δ
config/options.go	100% <100%> (ø)	⬆️
config/config.go	94.06% <100%> (+0.26%)	⬆️

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[yurishkuro]

Nice! Thanks!