Add an API to reset/unset options and expose it at CLI level

[mwestphal]

Is your feature request related to a problem? Please describe.
With the addition of optional options, some options are now unset by default.
It would be nice to be able to unset an option that have been set.
Not only at libf3d API level but also at CLI level in order to override a config file option with CLI.

Describe the solution you'd like

• Add options::unset specific to optional options
• Add options::reset that either unset or reset to default
• [maybe] add a options::isOptional
• In F3DStarter::UpdateOptions, detect "default" as a special string and in that case call options::reset instead of setAsString