feat(stage): add environment field

[ecrupper]

Related Issue

Adding an environment map to the stage environment. I've worked on the server functionality and did not end up using the MergeEnv function that I copied over from step, but I figured adding the same function to the stage object would be good for future use if the need were to arise.

[codecov]

Codecov Report

Merging #215 (7a91e85) into master (28210cf) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #215      +/-   ##
==========================================
+ Coverage   96.71%   96.73%   +0.02%     
==========================================
  Files          53       53              
  Lines        5595     5644      +49     
==========================================
+ Hits         5411     5460      +49     
  Misses        137      137              
  Partials       47       47              

Impacted Files	Coverage Δ
pipeline/stage.go	100.00% <100.00%> (ø)
yaml/stage.go	100.00% <100.00%> (ø)

[jbrockopp]

Could we add a test here for this YAML functionality?

Specifically, with regards to unmarshalling YAML for stages with an environment block?

I think updating the existing test would work so here are the locations for reference:

types/yaml/stage_test.go

Line 173 in 87c103b

func TestYaml_StageSlice_UnmarshalYAML(t *testing.T) {

https://github.com/go-vela/types/blob/feat/add-stage-environment/yaml/testdata/stage.yml

[jbrockopp]

LGTM