basenconverter: add options to constructor with 'negativeCharacter' option and make `setNegativeCharacter` protected so that it can't be mutated.  This should mean that values converted with a converter will always be able to convert back properly.
basenconverter: add options to constructor with 'negativeCharacter' option and make `setNegativeCharacter` protected so that it can't be mutated.  This should mean that values converted with a converter will always be able to convert back properly.
tobymackenzie/php-utils