feat(html-only-element): defaultBindingMode, coerce

[bigopon]

This PR follows #301 to enhance @bindable declaration for html only element with defaultBindingMode and coerce.

Usage

<template bindable='deposit:number & fromView, currency'>
  <label>
    Enter deposit amount (${currency}): <input type='number' value.bind='deposit' />
  </label>
</template>

It doesn't require aurelia/templating#558 but it can work automatically after that got merged.

• Demo

[bigopon]

the main PR that makes this compelling was closed, and enabled in plugin form. If anyone interested in this feature, please discuss at https://github.com/aurelia-contrib/aurelia-typed-observable-plugin