Improvements to `CliArgs`

Summary

My initial implementation of CliArgs was enough to used in the example application, but there are some improvements that should ideally be made (see #341). Here I list my ideas that I did not yet implement.

Description

• Use std::any instead of string for ArgValue representation. Check against specified type when setting the value. Might be better to use std::variant and limit possible input types.
• Allow for positional arguments (.name = {"pos"})
• Handle equal sign: --opt=arg (currently flag and value must be space-separated)

Other ideas

• Introduce an operator T function in ArgValue and ArgVector to allow implicit conversion