Add ability to automatically resolve DB::connection('mysql::write') to use writable database connection as well as DB::connection('mysql::read') instead of adding duplicate connection config.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A

## Current Behaviour

You are required to create separate database configuration if you need to force using either "read" or "write" mode as suggested in <http://laravel.io/forum/05-25-2014-read-write-mysql-connection-forcing-a-select-to-use-write-connection>.

## Proposed Behaviour

```php
DB::connection('mysql::write')->table('user')->get();
User::on('mysql::write')->get();
```

Both above code should force using "write" pdo connection.

## Limitation

* Depending on the use case, this could actually increase your database connection count as separate PDO connection is created for "mysql" and "mysql::write" connection name.

Signed-off-by: crynobone <crynobone@gmail.com>