Sanitizing options on Get() ensures that when someone asks
for a given option which may not be defined (`nosubject`, `nosender`),
we'll return a valid value (`'true'` or `'false'`) and not `''` (empty
string, undefined).
This way, users do not need to wonder if "nosender is not set" is
handled like "true" or "false" or in some 3rd way. They also don't need
to think about "how to unset this setting, now that I've set it to something".
Options will appear to have a default sanitized value no matter if
they've explicitly been set or not.
The `NoSender()` and `NoSubject()` getters are just there for
convenience, so that we won't need to do casting in other places.
The configuration setting is called `Hide*` instead of `Show*`, because
it's backward compatible with existing configuration settings.
This is useful for when you setup an email forwarding inbox and you're
always sending to it through the same email address. In that case, you
don't need to see the email address in each Matrix message.
In the future, another similar `bool` setting (`hide-subject`) will land,
which controls whether the email's subject is shown in the final message
or not. That setting can make use of most of the same setup (all of
`handleBooleanConfigurationKey`).
Slavi Pantaleev
