Add "check" as a configuration option and fail "ember test" if coverage fall below threshold

[ssylvia]

check: {
  global: {
    statements: 0,
    lines: 0,
    branches: 0,
    functions: 0,
    excludes: []
  }
  each: {
    statements: 0,
    lines: 0,
    branches: 0,
    functions: 0,
    excludes: []
  }
}