Fix mistake in usage comment.
Make sure usage works on Mac OS X
Add option to say "Yes to all".

Test plan:
1. echo 'hello world 1234' > file1.txt
2. echo 'hello world 5678' > file2.txt
3. ./codemod.py 'hello' 'bye' --extensions 'txt'
   -> selected "Yes to all" option
4. cat file1.txt
   -> 'bye world 1234'
5. cat file2.txt
   -> 'bye world 5678'