Set-PSReadlineOption -ExtraPromptLineCount does not work anymore in 2.0.0 Beta (master)

[nanoant]

This is follow-up of #470

Environment data

PS version: 5.1.15063.786
PSReadline version: 2.0.0-beta1
os: 10.0.15063.0 (WinBuild.160101.0800)
PS file version: 10.0.15063.0 (WinBuild.160101.0800)

Steps to reproduce or exception report

According to the documentation when using multi-line prompt, one needs to add Set-PSReadlineOption -ExtraPromptLineCount <NumberOfExtraLines>. While it worked in 1.2, it does not work anymore in 2.0.0 Beta (master), so Ctrl+L is removing first line of the prompt regardless of the setting.
Use following profile.ps1 to replicate the bug:

function Prompt {
    Write-Host $($ExecutionContext.SessionState.Path.CurrentLocation)
    return '>' * ($nestedPromptLevel + 1) + ' '
}
Set-PSReadlineOption -ExtraPromptLineCount 1