Fix explicitly false booleans

When a boolean option is explicitly set to `false`, e.g.:

```bash
--option=false
```

it enters slop as the string value `'false'`. This commit updates the
`BoolOption` option handler to interpret `'false'` and various other falsey
values (`false`, `'false'`, `'no'`, `'off'`, `'0'`) as logically false.

1 files changed, 3 insertions, 1 deletions

diff --git a/lib/slop/types.rb b/lib/slop/types.rb
index 4ccd42e..83e4a0b 100644
--- a/lib/slop/types.rb
+++ b/lib/slop/types.rb
@@ -21,6 +21,8 @@ module Slop
   class BoolOption < Option
     attr_accessor :explicit_value
 
+    FALSE_VALUES = [false, 'false', 'no', 'off', '0'].freeze
+
     def call(value)
       self.explicit_value = value
       !force_false?
@@ -35,7 +37,7 @@ module Slop
     end
 
     def force_false?
-      explicit_value == false
+      FALSE_VALUES.include?(explicit_value)
     end
 
     def default_value