Add support for loading arguments from a file

[marktoman]

Thank you for this amazing framework, first of all.

One general issue in the case of similar frameworks is specifying persistent values for parameters. Environment variables work well for ad-hoc runtime replacement, but if they are the only way to achieve this, an ugly starter script is required to run your app.

To solve this well, I propose adding support for Generic Host. The reason is that it has thought-through support for config files that can be easily overridden for development, production, and so on, and even by environment variables. I think it could live side by side with the current options, which are just great.

[Tyrrrz]

I think a better option is to add support for response files so that you can load arguments from a file.

[Tyrrrz]

I decided that loading arguments from a file is also too niche of a use case. You can plug CliFx into generic host instead, using code as examplified here: #22