[FIX] lang_LT, lang_LV: negative amounts (#185)

Negative amounts were not working (when no currency is used), because
`get_digits` method does not expect `-` sign, which crashes conversion.
To avoid that, we split minus sign from number string and prepare its
word to be used with amount words.

closes: #184

num2words/base.py

@@ -89,6 +89,13 @@ class Num2Word_Base(object):
 
             return out
 
+    def parse_minus(self, num_str):
+        """Detach minus and return it as symbol with new num_str."""
+        if num_str.startswith('-'):
+            # Extra spacing to compensate if there is no minus.
+            return '%s ' % self.negword, num_str[1:]
+        return '', num_str
+
     def to_cardinal(self, value):
         try:
             assert int(value) == value