title: Add a 'filepath' column to a pandas DataFrame
body: <p>i have a list of 100 or so jsons, which are being read, filtered, and appended into a pandas dataframe:</p>
  
  <pre><code>import pandas as pd
  import glob
  
  dfOutput = pd.DataFrame()
  for filepath in glob.iglob('/Users/vinceparis/dev/dfyb/dataset/cucumber_test/out/*.json'):
      dfRead = pd.read_json(filepath, orient='columns')
      dfFiltered = dfRead.filter(items=['label', 'confidence'])
      dfOutput = dfOutput.append(dfFiltered)
  print(dfOutput)
  dfOutput = dfOutput.to_csv('/Users/vinceparis/dev/dfyb/growlog2.csv')
  </code></pre>
  
  <p>the output will be a nice, single dataframe</p>
  
  <pre><code>         label  confidence
  0     seedling        0.33
  0     cucumber        0.35
  1   cotyledons        0.38
  0     seedling        0.36
  1   cotyledons        0.31
  2      flowers        0.38
  3      flowers        0.34
  0     cucumber        0.48
  ..         ...         ...
  0   cotyledons        0.41
  1   cotyledons        0.42
  0     cucumber        0.36
  0   cotyledons        0.43
  1   cotyledons        0.34
  0      flowers        0.36
  1      flowers        0.40
  </code></pre>
  
  <p>how might one add a column for 'filename', which would include the path of the original json which was poured into the dataframe?</p>