ENYO-700: Custom holdpulse configuration to account for various input devices.

### Issue
Various input devices have differing delays between when the `keydown` and `keyup` events are sent, causing the `holdpulse` event to sometimes be sent unintentionally.

### Fix
We use a custom holdpulse configuration for `Moonstone` to account for this variation, by introducing a new construct, the `init.js` file, as a central location for library-specific configuring. The initial delay for starting a hold has been increased to 400ms - this shouldn't have too much of an impact on user interaction, as there is an expected hold delay before an action repeats itself (versus, say, a normal button press, which is unaffected by this increase).

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>