Added handling for custom trigger button in theme
Added handling for custom trigger button in theme

refs https://github.com/TryGhost/membersjs/issues/8

This allows theme developers to use a custom trigger button in their theme instead of default UI created by script by marking the custom button with data attribute - `data-members-trigger-button` .

In case of a custom trigger button, CRA doesn't render the default trigger button and attaches click event handling to the custom button to control the popup UI. Script also adds new custom class `popup-open` and `popup-close` based on popup state to allow theme developers to handle button UI based on popup state.

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