Allow passing a param to change the filename of the conf file.

Hi, this PR allows you to pass in a filename if you want a different name for the configuration file. This allows you to set an order to the limits created.

Example:
```ruby
user_ulimit "tomcat" do
  filehandle_hard_limit 10240
  core_limit 'unlimited'
  filename '99tomcat'
end
```
Good call. Updated