bugfix: settings with invalid dimension should be ignore and warning should be issued

passing an invalid dimension should not treat the block as master block. the following setting should not be applied if `unknown` is not a valid dimension.

```
{
    settings: ["unknown:value"]
    some: "value"
}
```

/cc @davglass


OK, let's tear it out then. I added it while fixing issue #26 since the rest of the logic in this function was dropping "master" sections (since that doesn't match a defined dimension (by design)).