bugfix base64 file decoding

[OscarLuu]

This PR fixes a bug where the file opener tries to decode files into utf-8 when some files are not necessarily text files.

Opens it in binary mode and base64 encodes this.

Error:

"/home/local/python39_venv/lib/python3.9/site-packages/assetman/compilers.py", line 203, in replacer
    encoded = base64.b64encode(open(path).read())
  File "/home/local/lib/python3.9/codecs.py", line 322, in decode
    (result, consumed) = self._buffer_decode(data, self.errors, final)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0x89 in position 0: invalid start byte