create standalone decoder tool that takes bytes on stdin

I'd like to be able to query DR's decoder just like we can do with other decoders. E.g.:

$ echo ' 0xc4 0xe2 0x65 0x90 0x14 0x80 ' | capstone -x86 -
+0x0000  c4e265901480   vpgatherdd	ymm2, dword ptr [eax + ymm0*4], ymm3

We just need to make a front-end that parses hex strings and passes them to drdecode.

Unfortunately with today's build setup we can't easily have a single tool binary that decodes both ARM, AArch64, and x86.