Allow line-height in @page context

[bertfrees]

Use OBFL's row-spacing attribute on layout-master to implement it. Note that we should make sure OBFL's behavior is consistent with CSS (w.r.t. cascading etc.). This is not an essential feature because setting double line spacing document-wide can also be accomplished with:

* {
  line-height: 2;
}

See also:

• https://github.com/snaekobbi/braille-css-spec/issues/18
• https://github.com/snaekobbi/pipeline-mod-braille/issues/26#issuecomment-138556474