Updated update-docs command to respect mixed file type extensions

[jwlawrence]

• This is needed when working on transforms that require different i/o file types. Addresses update-docs command requires i/o to be matching filetypes #120
• The regex is used to only process files with 'input.' or 'output.' in their filename. For some reason lookbehind groups are deemed invalid by eslint, which should be supported in these v
  ersions of node, but I wasn't able to get eslint to accept it. It would provide an additional layer of accuracy by ensuring a '.input.' pattern, but I feel confident that the lookahead is
  safe to use given the relatively structured nature of fixture filenames.