text: Title: Support passing multiple input_file parameters to ``evaluate'' command
  Body: **Is your feature request related to a problem? Please describe.**
  I want to evaluate multiple datasets (same formatting, they can share the same dataset reader). The "evaluate" command takes much longer to load the model than to evaluate.
  
  **Describe the solution you'd like**
  support passing multiple input files and output files to the "evaluate" command
  
  **Describe alternatives you've considered**
  I saw this issue https://github.com/allenai/allennlp/issues/2663.  I'm not sure if anyone is working on it.
  
  
  