Use less hacky way of prompting y/n

A bug report indicated some odd behavior after being prompted y/n and hitting tab.
While I can't reproduce the problem, I suspect it has something to do with how
the prompt is being cleared.

After introducing the status line for Ctrl+R, I realized I could use that for
prompting y/n, so hopefully this change addresses the reported bug.
Fix exception with long status line

Long status lines (which can happen rarely on purpose but easily by accident)
were causing an exception in Render() because it assumed the status region
would only ever be 1 line.
Use case insensitive history search by default

PowerShell is mostly case insensitive, so history search should also ignore case.
There are uncommon situations where someone may want to use a case sensitive search,
so an option is provided to control the search.