Allow full-line highlights

I don't like signs (because even if they have no text, they seem to [force](http://vim.1045645.n5.nabble.com/Is-it-possible-to-hide-the-quot-sign-column-quot-td3214239.html) you to have a `SignColumn`) and column-range specific highlights don't always work.

This rough commit tries to solve the problem by adding a new flag, `g:syntastic_highlight_lines`, which highlights the whole offending line (minus trailing newline).

I feel this is a rather heavy handed way to achieve this goal, but maybe it is actually the right way and should be merged. Even if it isn't, at least it shows precisely what behaviour I want but I don't think is available with syntastic as-is.