HighLine：agree

>> require "highline/import"
=> true
>> agree("foo")
foo
y
=> true
>> agree("foo")
foo
yes
=> true
>> agree("foo")
foo
no
=> false
>> agree("foo")
foo
n
=> false
>> agree("foo")
foo
a
Please enter "yes" or "no".
foo
yes
=> true
>> agree("foo", false)
foo
y
=> true

character は false を与えるものではない気がするが…