Add rule: declarations-before-nesting

[onishiweb]

This rule should enforce that all declarations in a block should come before nesting

This:

.selector {
  property: value;

  .nested-selector {
    property: value;
  }
}

Not:

.selector {
  .nested-selector {
    property: value;
  }

  property: value;
}

Discussed in: Financial-Times/origami-build-tools#268

[DanPurdy]

@onishiweb thanks for this, I'll hopefully find some time this weekend to go over this and hopefully pop it into our next release. Busy times at moment!