Feature: Make commit status context configurable

[HaoTanAtMLB]

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]
])