Validations on parameters values and allow multiple commands on the :vf key

This pull request has two parts.

1. Allow multiple commands on the  `:vf` key. 
    * I came into this _issue_ trying to get a thumb image (jpg) from a video and getting it rotated too. This wasn't posible because in the _pad_only_ process  the `:vf` key was being overwritten. 
    * __Example:__
        * Before:
            * `-vf transpose=1` <- This command will be overwriting.
            * `-vf scale=250:-1,pad=250:250:0:55.0:black`
        * Now:
            * `-vf scale=250:-1,pad=250:250:0:55.0:black,transpose=1`
2. Validations on parameters values.
    * In some cases some keys may come with an empty string or nil.