Add ability to black list selectors about postcss-pxtorem

[mars-abd]

It will be good to have ability to ignore some properties locally (not all properties by name), may be by comments, I don't know how

[cuth]

Right now a good trick is to use an uppercase letter in the "px" since the match is case sensitive.

a {
  font-size: 12Px;
}

This would be ignored by pxtorem and still work in all browsers.

[mars-abd]

It's a little bit unobviously, but it is good!
I think it should be added to readme docs.

[cuth]

You are correct. That needs to happen.

[cuth]

I've added a paragraph to the README about ignoring declarations with uppercase px.