[#375] Only return expression length to stop annoying whole block underlining in vs code
[#375] Only return expression length to stop annoying whole block underlining in vs code

While developing you often want to select one set of tests to run. In vs code because mocha-avoid-only returns the entire length of the block, every single line gets underlined, making it difficult to ignore and dev against.

closes #375

Microsoft/tslint-microsoft-contrib,Microsoft/tslint-microsoft-contrib