outline-counter: style numbering

[uliska]

In the text.outline-counter module (#7) the numbering style can be configured for all levels with

\createOutline foo number.number.alpha-lower

However, while the text of an outline item can be styled in any way using regular markup commands the outline number is fixed to plain roman text. There should be a way to assign a (series of) markup command(s) to be applied to the result of \inc with, say:

\createOutline \with {
  numbering-style = #'(sans bold italic)
} foo number.number.alpha-lower

Ideally this could even be made to work on individual levels so that

\createOutline \with {
  numbering-style = 
  #'(
     (sans bold italic)
     (italic))
} foo number.number.alpha-lower

would apply \sans \bold \italic to the first-level outline number and \italic for the remaining ones.

The optional argument should be implemented using the \with-options macro from oll-core. Further configuration options could be added in the future.