Support transparent decompression of UPX'd binaries

[tstromberg]

Similar to how we transparently decompress other archival formats, we should be able to run "upx -d" if a program appears to be compressed with UPX.

We should probably show the decompressed payload as if it existed "inside" the other one, so that the rule matches for both compressed and uncompressed payloads show up differently. For example:

# file
- HIGH: this file was compressed with UPX!

# file ∴ upx
- CRITICAL: it's malware!

If a UPX'd binary is encountered, but the upx utility is unavailable, an error message should be sent to stderr, saying something like:

"upx" not in path, unable to decompress binary for further analysis