Feature: Add command for listing features

[alistairjevans]

New command for listing the set of features in a project

Example usages:

Simple

> autostep list features
 Feature 1
 Feature 2

Scenarios

> autostep list features --with-scenarios
 Feature 1
     Scenario 1
 Feature 2
     Scenario 1

JSON

> autostep list features --json
{
  "features": [
     { "name":  "Feature 1", "path": "./feature1.as" },
     { "name":  "Feature 2", "path": "./feature2.as" }
  ]
}