Add "it's" to E-Prime

Author: jdkato

features/rules.feature

@@ -12,6 +12,7 @@ Feature: Rules
     Then the output should contain exactly:
       """
       test.md:3:1:write-good.E-Prime:Try to avoid using 'Here's'.
+      test.md:6:1:write-good.E-Prime:Try to avoid using 'It's'.
       """
 
   Scenario: Repeated words

fixtures/E-Prime/test.md

@@ -1,3 +1,6 @@
 # `write-good.E-Prime`
 
 Here's a new book.
+
+
+It's a good idea.

write-good/E-Prime.yml

@@ -16,6 +16,7 @@ tokens:
   - i'm
   - is
   - isn't
+  - it's
   - she's
   - that's
   - there's