#5 Span tool

[Daniel Polansky]

Proposal: Span tool should be added, adding the "span" element to the selection, or removing it if already present. If the "span" attribute is not present, at least the style attribute of the span element should be editable by the user, in a simple dialogue.

Menu: "Format".

Menu item: "Span".

The feature is only partially implemented in Kafenio.