Store multiple hosts info per `--aws_key`

ec2ssh can, so far, hold only a setting though user would pass `--aws_key`s. This pull request allows us to store multiple hosts information per `--aws_key` like below:

```
### EC2SSH BEGIN ###
# Generated by ec2ssh http://github.com/mirakui/ec2ssh
# DO NOT edit this block!
# Updated 2013-04-15T17:24:27+09:00

# section: foo
Host foo-app001.ap-northeast-1
  HostName ec2-1-1-1-1.ap-northeast-1.compute.amazonaws.com

...

# section: bar
Host bar-1.us-west-1
  HostName ec2-2-2-2-2.us-west-1.compute.amazonaws.com

...

### EC2SSH END ###
```