adding "selected currencies" reactive var example

[ivnnv]

Hi,

As proposed in this #361 issue by @wtrocki, I'm adding a minimal example of having a reactive variable that stores the selected values from the currencies list.
The final goal of this demo is find out if this library could be to have a way to allow persistence to a given set of reactive variables and so being able to retrieve the stored value on app initialization (like on a page refresh, then the user will still have the same selection that had before refreshing the page).

Hope this helps.

PS: Thanks to the apollo team for creating all of this goodness.

[apollo-cla]

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