[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

tests/test_lt.py

@@ -44,6 +44,14 @@ class Num2WordsLTTest(TestCase):
             "aštuoni šimtai dvidešimt du trilijonai aštuoni šimtai dvidešimt "
             "keturi milijardai trys šimtai aštuoniasdešimt keturi milijonai "
             "du šimtai dvidešimt tūkstančių du šimtai devyniasdešimt vienas")
+        self.assertEqual(
+            num2words(-5000, lang='lt'),
+            'minus penki tūkstančiai',
+        )
+        self.assertEqual(
+            num2words(-5000.22, lang='lt'),
+            'minus penki tūkstančiai kablelis dvidešimt du',
+        )
 
         # print(fill(n2w(1000000000000000000000000000000)))
         # naintilijonas