Add clearableValue option for Multi-Select

[slybridges]

Fixes #458

[slybridges]

On this PR I've added a note in the README that the clearableValue should only be used along with the clearable prop set to false.

This is because if we want not to clear the options that have clearableValue=false when pressing the Clear Allbutton, we have to iterate over each valueArray all the time which I felt could have some perf issues.

[slybridges]

@JedWatson - anything you'd need from me to get this merged ?

[JedWatson]

Sorry for the delay @slybridges - this looks great, I'm merging it now.

[slybridges]

👍 @JedWatson.
No worries for the delay, I understand open source can be tough on you sometimes ;)

[shivamjjha]

Seems this is removed in latest version of react-select ?
https://codesandbox.io/s/codesandboxer-example-forked-pn5c9c?file=/example.tsx

[vivekrekhadia]

Seems this is removed in latest version of react-select ? https://codesandbox.io/s/codesandboxer-example-forked-pn5c9c?file=/example.tsx

How you solved it then, I am having the same issue.