Add support for span tags

[axw]

Add elasticapm.SpanContext.SetTags method.

Closes #147

[codecov-io]

Codecov Report

Merging #213 into v2 will decrease coverage by <.01%.
The diff coverage is 79.31%.

@@            Coverage Diff             @@
##               v2     #213      +/-   ##
==========================================
- Coverage   80.42%   80.41%   -0.01%     
==========================================
  Files          87       87              
  Lines        5190     5219      +29     
==========================================
+ Hits         4174     4197      +23     
- Misses        818      821       +3     
- Partials      198      201       +3

Impacted Files	Coverage Δ
module/apmot/span.go	86.87% <100%> (+0.16%)	⬆️
model/marshal_fastjson.go	86.69% <77.77%> (-0.27%)	⬇️
spancontext.go	91.66% <77.77%> (-8.34%)	⬇️
tracer.go	83.92% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a640e65...f969887. Read the comment docs.