Detect encoding of clamav process output and decode accordingly

[tohoku]

#142 mentions the following error getting returned intermittently:

[ERROR] UnicodeDecodeError: 'utf-8' codec can't decode byte 0xe0 in position 276: invalid continuation byte
Traceback (most recent call last):
File "/var/task/scan.py", line 236, in lambda_handler
scan_result, scan_signature = clamav.scan_file(file_path)
File "/var/task/clamav.py", line 195, in scan_file
output = av_proc.communicate()[0].decode()

Challenge: The av_proc.communicate() method returns the output of the clamscan call, but sometimes that output is using a different charset than the default utf-8 used by decode().

As a workaround, this PR will use chardet to try to determine the charset and decode accordingly.

In issue #183, removing the -a flag from the clamscan call is mentioned as a possible workaround, but I have not tried this.

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