Add linter for CHANGELOG formatting
Add linter for CHANGELOG formatting

There have been a number of recent commits introducing incorrectly
formatted CHANGELOG entries:
- 9f0b8eb was missing an author
- 936a862 had trailing whitespace
- 238432d had wrong number of leading whitespace
- 51852d2 had wrong number of leading whitespace in the header

To prevent these inconsistencies from happening in the future, I wrote a
small linter for CHANGELOG files that catches all of the above errors.

rails/rails,Erol/rails,BlakeWilliams/rails,georgeclaghorn/rails,yahonda/rails,MSP-Greg/rails,esparta/rails,Erol/rails,rails/rails,georgeclaghorn/rails,MSP-Greg/rails,BlakeWilliams/rails,BlakeWilliams/rails,esparta/rails,esparta/rails,rails/rails,Erol/rails,BlakeWilliams/rails,yahonda/rails,MSP-Greg/rails,yahonda/rails,esparta/rails,MSP-Greg/rails,Erol/rails,georgeclaghorn/rails,rails/rails,georgeclaghorn/rails,yahonda/rails