Should --get throw a warning if a variable is unknown

[jacobmealey]

currently if you call trurl --url "https://example/com" -g "{host} {foo}" it will print example . I think it should either print example {foo} or example to stdout and some type of warning to stderr that says "Unkown variable foo". I'm leaning towards the warning to stderr and using trurl_warnf like in #231.

[bagder]

A third option is to error out completely, to make it more apparent if you spelled a keyword wrong etc...

I have a feeling we are not going to add many more keywords going forward so there is little risk that you're going to end up in a situation where you run an older version using a newer command line and thus using a keyword not yet implemented.

[jacobmealey]

Okay, that makes sense. 👍

[bagder]

See #286