allow easy modifications of ffmpeg command line. fixes #23

Separated argument generation and stuck the results in a control so advanced users can tweak things if necessary.

The behavior is that if you change a parameter that results in a new argument string, the argument textbox gets overwritten. When you hit "convert", it uses whatever's in the textbox.

![image](https://cloud.githubusercontent.com/assets/211868/2662740/8c00b31c-c044-11e3-8dda-22099df852d2.png)