Added VPC support

- Added: Optional parameter "--dns-name-key"
 - for get private host name from VPC (no global IP) instance.
- Changed: 'Name' tag + instance id is used as Host description. 
 - because  set same 'Name' by Auto Scaling Group.  
- Added: ssh_config_keywords in dotfile 
 - because set 'ProxyCommand' after Host description

````
Host XXXXXXXXXXXX_i-1XXXXXXX.ap-northeast-1
  HostName ip-XXX-XXX-XX.ap-northeast-1.compute.internal
  ProxyCommand ssh -o ServerAliveInterval=60 XXXXXXXXXX %h %p
````

# How to Use 

## non VPC
 - (same as before)
## VPC
 
### 1. Append bastion server information to .ec2ssh

example

````
path: /path/to/ssh_config
aws_keys:
  default:
    access_key_id: YOUR_ACCESS_KEY_ID
    secret_access_key: YOUR_SECRET_ACCESS_KEY
regions:
  - ap-northeast-1
ssh_config_keywords:
  - ProxyCommand ssh -o ServerAliveInterval=60 YOUR_BASTION_SERVER nc %h %p
````

### 2.  Execute ec2ssh update with "--dns-name-key private_dns_name" option

````
$ ec2ssh update --dns-name-key private_dns_name
````