Enable setting the phpcs standard through .vimrc

Hi,

First off, thanks for an awesome plugin!

I thought that it would be nice to be able to let vim decide which standard to use and not be dependent on the global `phpcs --config-set default_standard`, thus this PR.

Changes to the standard can be done with
```
let g:php_phpcs_checker_standard="PSR2"
````

I'm looking forward to your comments.