Add alias support

[ej-shafran]

• Add aliases and alias_count fields to the Flag struct
• Respect the aliases when parsing flags
• Print out aliases when printing out options
• Update example to feature this

[rexim]

@ej-shafran this is a cool idea. Please don't close this PR yet. I'll get back to it when I get a chance and fix the conflicts myself.

[ej-shafran]

@rexim inspired by the struct-as-named-params thingie you did in Nob in recent streams, I think something like that could be nice for this feature/this library in general