Adds environment set up; Adds executor tests

[cirocosta]

closes #3
closes #6

Aside from covering those issues, it also makes possible to perform templating on any of those fields. For instance:

Jobs:
  - Id: 'Job1'
    CaptureOutput: true
    Run: 'echo lol'

  - Id: 'Job2'
    CaptureOutput: true
    Env: 
      FOO: '{{ .Jobs.Job1.Output }}'
    Run: 'echo $FOO'
    DependsOn: [ 'Job1' ] 

would lead to

lol
lol

---

Sample execution:

Env:
  WORD: 'BAZ'

Jobs:
  - Id: 'SayFoo'
    Run: 'echo $WORD'
    Env:
      WORD: 'foo'

  - Id: 'SayBaz'
    Run: 'echo $WORD'
    DependsOn: [ 'SayFoo' ]

  - Id: 'SayCaz'
    Run: 'echo $SAY'
    Env:
      SAY: 'CAZ'
    DependsOn: [ 'SayFoo' ]

leads to

cr --file ./examples/environment.yaml --stdout=true

	Starting execution.

	Logs directory:	/tmp
	
SayFoo	status=STARTED	start=21:22:55
foo
SayFoo	status=SUCCESS	start=21:22:55	elapsed=4.176744ms
SayBaz	status=STARTED	start=21:22:55
SayCaz	status=STARTED	start=21:22:55
BAZ
SayBaz	status=SUCCESS	start=21:22:55	elapsed=4.781021ms
CAZ
SayCaz	status=SUCCESS	start=21:22:55	elapsed=5.706108ms