Vows filtering

While it’s not quite “run a single vow from command line” or the `@focus` decorator, it’s a step in that direction. I’ve added the ability to filter out vows and contexts that match a regex pattern, via a `-e` command-line switch.

**Example usage:**

    (pyvows)
    oystercar ☃ ~VIRTUAL_ENV ➤ 3488b87|vows-filtering ಠ_ಠ
    10094 ± : pyvows -e test                                           [14m] ✹ ✭
    
     ============
     Vows Results
     ============
    
      ✓ OK » 499 honored • 0 broken (0.271766s)
    
    (pyvows)
    oystercar ☃ ~VIRTUAL_ENV ➤ 3488b87|vows-filtering ಠ_ಠ
    10095 ± : pyvows -e '[Tt]est'                                      [14m] ✹ ✭
    
     ============
     Vows Results
     ============
    
      ✓ OK » 364 honored • 0 broken (0.157240s)
    
    (pyvows)
    oystercar ☃ ~VIRTUAL_ENV ➤ 3488b87|vows-filtering ಠ_ಠ
    10096 ± : pyvows -e '[Tt]est' -e 'Should'                          [15m] ✹ ✭
    
     ============
     Vows Results
     ============
    
      ✓ OK » 364 honored • 0 broken (0.157615s)

    pyvows -e should
    
     ============
     Vows Results
     ============
    
      ✓ OK » 130 honored • 0 broken (0.252356s)
    
    
    (pyvows)
    oystercar ☃ ~VIRTUAL_ENV ➤ 3488b87|vows-filtering ಠ_ಠ
    10097 ± : pyvows -e '[Tt]est' -e '[sS]hould'                       [15m] ✹ ✭
    
     ============
     Vows Results
     ============
    
      ✓ OK » 127 honored • 0 broken (0.147614s)