Consider separate 'Cleared' and 'Unset' values for badges.

[fallaciousreasoning]

This is semi related to #19.

What the behavior of Badge.clear({ scope: '...' }) should be is a little unclear.

Is the badge for that specific scope now clear or does it display the badge from a parent scope?

Perhaps we should remove separate set/clear methods in favor of something like this:

Badge.set({
    badge: number | boolean | undefined,
    scope: string
});

A badge for a scope can be in one of four states:

• unset: The badge from a parent scope will be applied (or none, if there is no badge for a parent scope).
• cleared: No badge is displayed for the scope.
• flag: A special value indicating a notification dot should be displayed.
• <number> A non-zero, positive number: The value will be displayed in the badge.

If badge is undefined or not present then the badge for the scope is unset
If badge is a boolean false sets the badge to cleared while true sets it to flag.
If badge is a number 0 sets the badge to cleared while other positive numbers set the badge to that number.

Alternatively, we could introduce separate methods for each value, as in #19.

// 0 sets the badge to cleared, other values set the badge to that value.
Badge.setNumber(<number>, { scope: '...' }); 

// false sets the badge to cleared, true sets the badge to flag.
Badge.setFlag(boolean, { scope: '...' });

// Unsets the badge.
Badge.unset({ scope: '...' });

I think this second approach is more friendly for polyfilling (though both should be polyfillable) if we add a new type (such as glyphs). However, in the second approach, it's less obvious that there is only one badge value active at any time.

@domenic I know you had some thoughts on this.
@marcoscaceres @mgiuca @raymeskhoury

[fallaciousreasoning]

@raymeskhoury floated that we stick with the current Badge.clear({ scope: '...' }) design and add an optional extra noInherit property if we decide we need it in the future.

Badge.clear({
    scope: `/special-scope`,
    noInherit: true
});

which would make the badge for /special-scope not check parent scopes for badges. So maybe we don't need to do anything here.

[fallaciousreasoning]

Resolution on this was that we would start with just unset and (possibly) add a separate cleared value in future, if it seemed like it would be useful. However, with the changes proposed in #55 this will no longer be needed.