Warning for disable directives that do nothing.

[BobCga]

It should be possible to detect and report when a disable directive is doing nothing. There are at least two ways to approach this:

1. When a shellcheck disable directive is present, but the disabled check isn't raised by the following line (or in the file when the directive is at the top), then emit a warning something like:

   "Line nn: 'SCnnnn' disabled but not raised. This shellcheck directive should be removed."

2. Provide a shellcheck option to ignore or validate disable directives.

[matthewpersico]

+1 - I was just about to enter a request for this, but github is apparently beta-ing a feature that pops up possibly-related issues to what you type for an issue title and this popped. Nice github feature. Please consider warning on directives that may no longer apply to the code that follows them. I've been going back and correcting code and forgetting to whack the directives.