[Drop-down-menu] Added multiple items selection support

[xmityaz]

User can set bool property "isMultiple" to make drop-down multiselectable.

Some screens attached bellow

1. Multiselectable menu:
   
2. Closed drop-down with several selected items
   

[oliviertassinari]

bool

[xmityaz]

@oliviertassinari thanks for your review. Mostly there are typos, fixed in next commit.

[oliviertassinari]

@xmityaz If you can squash the 4 commits into one, it's event better

[oliviertassinari]

Suggestion : https://github.com/airbnb/javascript#13.2

[xmityaz]

Yes, it would be better to follow same approach as in the rest of code.
Thanks

[xmityaz]

@oliviertassinari unfortunately I can't squash first 2 commits. It were merge from original base. I think will be easier to fork repo again and push all changes in 1 commit, then just make new pull request.

[jkruder]

As part of the data table #890 I added the capability to select multiple entries. The multi-select works almost identically to Excel's. We should look at abstracting this logic so that it can be shared between these components and others (the new list component comes to mind).

[hai-cea]

Thanks @xmityaz @oliviertassinari @jkruder multi item selection will be available on the new menus that will be released soon. It works much like how selects work, by passing an array of values into the value prop.