Policy: double quote, allow if dollar in string (#340)

Author: tinmarino

vint/linting/policy/prohibit_unnecessary_double_quote.py

@@ -11,6 +11,7 @@
 # see `:help expr-string`
 _special_char_matcher = re.compile(
     r'(\'|'  # allow single quote
+    r'\$|'  # allow dollar
     r'\\('  # prefix back slash
     r'(?P<octal>[0-7]{1,3})'
     r'|(?P<hexadecimal>[xX][0-9a-fA-F]{1,2})'