Better solution to ignore properties

I appreciate the [mention of a workaround in the docs](https://github.com/cuth/postcss-pxtorem#a-message-about-ignoring-properties) but I wonder if something like this would be a better pattern, similar to the behavior used by `eslint` (i.e. with a comment):

Input:

```
h1 {
font-size: 48px;
}
h2 {
/* pxtorem-disable-next-line */
font-size: 24px;
}
```

Output:

```
h1 {
font-size: 3rem;
}
h2 {
font-size: 24px;
}
```

I haven't looked at the source yet to see what kind of lift this would be, but would you accept a PR with this kind of solution?

Cheers, and thanks for the great library
Jeremy