Add customizations for cloudsearchdomain command

* Rename --query to --search-query to avoid shadowing our --query arg
* Add validation to require --endpoint-url when using cloudsearch domain
  commands.
Update changelog with latest changes
Bumping version to 1.3.20
Merge branch 'release-1.3.20' into develop

* release-1.3.20:
  Bumping version to 1.3.20
  Update changelog with latest changes
  Add customizations for cloudsearchdomain command
Don't validate --endpoint-url for help subcommand

"aws cloudsearchdomain" has --endpoint-url validation because
it's a required option for that service.  However, we should
*not* be validating this when a user is just trying to get
help.

The fix is to move the validation to the operation-args-parsed.*
event, which is after all the help processing code.

I've also added integration tests that actually run the help command
for cloudsearchdomain to verify this behavior works.
Merge branch 'fix-csdomain-help' into develop

* fix-csdomain-help:
  Don't validate --endpoint-url for help subcommand
Update changelog with issue 828