Add support to "noqa" option

[pricco]

I think it would be helpful to support a skip match parameter:

{skip: /noqa/}

[rlopezcc]

Combining the two matches sounds like a headache, because of how the current code is structured, and could be confusing. The same effect could be achieved using lookarounds.

I've been working on this patch, which allows you to add an arbitrary function in order to invalidate a match based on the line string or file-wide contents (this contemplates the case of file-level noqa).