feat: support assigned options

[sigoden]

The asssigned option must be assigned a value using the =, the name of asssigned option must have ':` suffix.

# @option --oa:
# @option --ob
# @option --oc:*

--oa: and --oc:* both have : suffix, they are assigned options, but --ob are not.

prog --oa=v1
prog --oa v1  #  error
prog --oc v1

A realworld example is tail -f/--follow

# @option -f --follow:[name|descriptor] <KIND?>

tail -f <path>
tail --follow <path>
tail --follow=name <path>