Add support for contrast detection

[Synchro]

I'd like Bootstrap to be able to accept radical colour palette changes without becoming unusable, for example to be able to switch to a white-on-black scheme simply by doing this:

@bodyBackground:       @grayDark;
@textColor:            @white;

At present there are lots of things that fail with this change because they are set to explicit colour values that don't provide sufficient contrast, and in some places even this is compromised (for example bootstrap occasionally uses white on yellow, and tries to work around it by using a particularly dark yellow). While I'm mainly thinking of this for aesthetic reasons, it would probably be very helpful for sites that need to conform to ARIA/WCAG standards that stipulate particular levels of contrast.

In order for this to be possible, there needs to be a way of dynamically selecting colours according to their contrast, but at present LESS lacks such a function. Fortunately I've written one, though it's been sitting in a pull request for some time, but when it's in, I'd like this to be considered for bootstrap.

[mdo]

Not something we'll add to the core for now. Thanks though.