added skip build feature

This makes it possible for the user to skip the build by passing configurable phrases as a comment on the pull request body.This would be helpful in resource constraint environment where in user doesn't want to trigger longer running builds for each commit/pull request. And also user might not want to trigger a build for minor patches like fixing 'typo'. If required, user can revert back by editing/removing the phrases from the pull request body to trigger a build.

This should match Travis ci, and be "[skip ci]"