Feature: Make commit status context configurable

The Github commit status is currently hard coded to `test-coverage-plugin`. It would be great if that is a optional parameter when calling the plugin. For example:

```
step([
$class: 'CompareCoverageAction',
jacocoCounterType: 'INSTRUCTION',
publishResultAs: 'statusCheck',
statusContext: 'my-custom-commit-status-context',
scmVars: [GIT_URL: env.GIT_URL]
])
```