Allow setting base style to the next match

[brechtm]

In some situations, it can be useful to create a new style to override a single style attribute. The other style attributes should not be affected by the new style. This can be achieved by setting the base style to the next match. A good example of this is forcing page breaks on arbitrary flowables (#186).

[page-break-paragraph : Flowable(has_class="page-break")]
base = NEXT_MATCH
page_break = any

Additionally, it would be helpful if the style log would also (recursively) list the base styles of the matched style.

[brechtm]

Implemented in 249d783