This patch implements FromIterator<&Style> and FromIterator<T:
Into<Style>> for Style to make it easier to programatically create
styles. Style already has a merge method, but it takes a slice instead
of an iterator.
As starting to use the cursive library I've found it challenging to
figure out how to actually use the themes. Only after checking the
source code, and another library actually using this feature, I've
managed to apply it to my project.
This change makes it easier for new commers to use toml themes by
extending the documentationn