Set options in multi-select filter dropdowns to selected initially.

[samussiah]

Related to #254. Currently all options in multi-select filter dropdowns are not selected even though the corresponding filter values are represented in the data.

[samussiah]

Probably just update this line from:

        .property('selected', d => d === control.start);

to

        .property('selected', d => d === control.start || control.multiple === true);