made addition phantomjs arguments configurable through yaml file.

The following configuration example does not work.
```yaml
rasterizer:
  command: 'phantomjs --proxy=myproxy:1234'
```
It causes:
```log
phantomjs failed; restarting
[uncaughtException] { [Error: spawn EMFILE] code: 'EMFILE', errno: 'EMFILE', syscall: 'spawn' }
```
made addition phantomjs arguments configurable through yaml file.