Support preset option values

[clue]

We should add support for option values that accept only a limited preset like this:

add <name> [--sign=(true|false|if-asked)]

Should parenthesis be optional? Probably easier to distinguish this from #5.

Depends on #3

[clue]

Parentheses are actually required in the first example, in order to not confuse the following alternatives with static keywords:

add <name> [--sign=(true | false | if-asked)]
add <name> [--sign=true | false | if-asked]

However, for optional values they are in fact optional because they're unambiguous:

add <name> [--sign[=(true | false | if-asked)]]
add <name> [--sign[=true | false | if-asked]]