refactor some code

[anshulxyz]

• Q. Why am I using isinstance() here:

  output = fixer(base, target, amount)
  if isinstance(output, float):
      output = "{} {} = {} {}".format(amount, base, output, target)
  
  click.echo(output)

• A. Because when the currency is invalid the operation is returning a string.

  except KeyError:
      result = 1.00 if base == target else "KeyError: Invalid curreny"
  
  return result

• Add newline at the end of the JSON files that are being written. Such as here

• Look for a better way to handle Connection Error Exception.

• Refactor file_handling.py