Add declarative syntax to menu items in fab-*-buttons

[bengry]

This PR aims to add a declarative syntax to all fab-*-buttons components (fab-default-button, fab-action-button etc.) for declaring menu items:

<fab-default-button text="Contact me">
  <contextual-menu-item key="email" text="Email" (click)="sendEmail()"></contextual-menu-item>
  <contextual-menu-item key="phone" text="Phone" (click)="call()"></contextual-menu-item>
</fab-default-button>

Also addresses <contextual-menu-item> directive items not having a comparable declarative syntax for their render and renderIcon inputs counterparts in IContextualMenuItem. Note that the overriding of an icon render doesn't work due to a bug in office-ui-fabric-react.

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Related issue: #3