[WIP] Add codecov.io (optional) integration

[goanpeca]

Fixes #417

• [ ] This is missing tests
• [ ] missing codecov.yaml, example

# https://codecov.io/gh/lektor/lektor/
comment: false
coverage:
  precision: 2
  range:
  - 70.0
  - 100.0
  round: down
  status:
    changes: false
    patch: true
    project: true

[PsiACE]

It's nice to see these changes. You can carry on with the work if you like.