Act env (#417)

* Test more if env variants

* The correct negation syntax is !=

* Make the Interpolate function support negated booleans from envs

* Move assert := a.New(t) into t.Run

This uncovered that some of the test premisses was wrong and the
Eval Bool function also had flaws

* Remove a stray logrus import

* Add an ACT env set to true

This can be used to skip certain steps that you don't want to run locally
when testing. E.g. steps that sends messages to Slack channels on successful
builds etc.

* Add a description about env.ACT to the readme

* A new attempt at Interpolation and EvalBool

* One small merge fix

* Remove some fmt.Printfs

* Fix some merge conflicts

README.md

@@ -166,6 +166,18 @@ Additionally, act supports loading environment variables from an `.env` file. Th
 act --env-file my.env
 ```
 
+# Skipping steps
+
+Act adds a special environement variable `ACT` that can be used to skip a step that you
+don't want to run locally. E.g. a step that posts a Slack message or bumps a version number.
+
+```yml
+- name: Some step
+  if: ${{ !env.ACT }}
+  run: |
+    ...
+```
+
 # Events
 
 Every [GitHub event](https://developer.github.com/v3/activity/events/types) is accompanied by a payload. You can provide these events in JSON format with the `--eventpath` to simulate specific GitHub events kicking off an action. For example: