add reduce_attributes flag

controls if attributes with empty value will collapse down to a boolean attribute
fix producing invalid html with reduce_attributes

Attributes with an equal sign but no actual value, either quotes or non quotes, are invalid.
Therefor when reduce_attribute is set to False we must add empty quotes to an empty attribute.
rename reduce_attributes to reduce_empty_attributes

which is a better descriptive name for what the properly does.
Merge pull request #6 from trbs/master

add reduce_attributes flag