Support environment variables in .ec2ssh

Hi!!

This patch allows users to use environment variables in .ec2ssh as follows.

```
$ cat ~/.ec2ssh
---
path: /home/yourname/.ssh/config
aws_keys:
  default:
    access_key_id: <%= ENV['AMAZON_ACCESS_KEY_ID'] %>
    secret_access_key: <%= ENV['AMAZON_SECRET_ACCESS_KEY'] %>
regions:
  - ap-northeast-1
```