FileContains does not support binary files

The code in FileContains.match():

        f = open(path)
        try:
            actual_contents = f.read()
            return self.matcher.match(actual_contents)
        finally:
            f.close()

opens a file in text mode on Python 3. From open()'s docstring:

  The default mode is 'rt' (open for reading text)
  ...
  Python distinguishes between files opened in binary and text modes,
  even when the underlying operating system doesn't. Files opened in
  binary mode (appending 'b' to the mode argument) return contents as
  bytes objects without any decoding. In text mode (the default, or when
  't' is appended to the mode argument), the contents of the file are
  returned as strings, the bytes having been first decoded using a
  platform-dependent encoding or using the specified encoding if given.

I think it would be kess surprising to open the file in binary mode.

An optional user-defined encoding might be useful too, if text is what
you need.