fix #121 and #123 - add support for reading env variables from .env (#133)

pkg/runner/testdata/secrets/.env

@@ -0,0 +1,3 @@
+MY_SECRET=top-secret
+MULTILINE_SECRET="foo\nbar\nbaz"
+JSON_SECRET={"foo": "bar"}

pkg/runner/testdata/secrets/push.yml

@@ -7,3 +7,7 @@ jobs:
     steps:
       - run: |
           echo '${{secrets.MY_SECRET}}' | grep 'top-secret'
+      - run: |
+          echo "${{secrets.MULTILINE_SECRET}}" | wc -l | grep 3
+      - run: |
+          echo '${{secrets.JSON_SECRET}}' | grep "{\"foo\": \"bar\"}"