Add ability to display number of items under group header

[jiayingxu]

Add ability to display number of items under group header using numMembers property

Example usage:

[kpfefferle]

Design question: should the continent names here be left-aligned? It appears from the screen capture that they are centered within an area of varying width which would likely lead to them not really looking aligned at all?

[jiayingxu]

I fixed the alignment in the screenshot. The screenshot is just of a demo I created locally where I hard-coded the number of items under the header with the countries label in select-item-layout. Overriding the default layout would require creating a view that overrides select-option.

[mixonic]

I think these are valid questions for the use case, but I'm not sure if they apply here in particular. Just passing the number through seems fine:

• The alignment should probably also be stable regardless of collapsed/expanded? So if you click the collapse toggle the test won't jump around?
• What does this look like if the group name is long enough to overlap the count?