Disabled items can't be selected

[reinert]

This is an initial proposed solution for #99.

Basically it prevents select method to set a disabled item as selected. Also selectNext and selectPrevious are covered: they try to find the nearest enabled item. These three methods now return true if an item was selected or false otherwise.

Not covered situations yet:

• When selected property is manually changed
• When a selected item becomes disabled

[reinert]

Also, should there be a feature toggle like skipDisabledItems allowing the user to decided whether disable items should be selectable or not?

[bicknellr]

Hey, just wanted to let you know this PR hasn't gone unnoticed and we'll get to it soon. Ping me here if 'soon' stops being accurate.

[reinert]

Ok. I will.

[reinert]

Idea: provide an attr-for-disabled option so the user can determine what attribute will be queried to check the disabled state. Default value, obviously, would be 'disabled'.

[bicknellr]

btw, I was poking around in paper-radio-group and found that it also implements this: https://github.com/PolymerElements/paper-radio-group/blob/3ee7a79/paper-radio-group.html#L164

[reinert]

Good!