Support binary target_obj in scratch creation

[encounter]

Also support --disassemble= in diff_flags, so the scratch can be limited to a single function within the object.

Example:
object.zip

curl -v http://127.0.0.1:8000/api/scratch \
  -F compiler=mwcc_242_81 \
  -F platform=gc_wii \
  -F compiler_flags=-lang=c++ \
  -F diff_label=__copy_longs_unaligned \
  -F target_obj=@mem_funcs.o \
  -F 'diff_flags=["--disassemble=__copy_longs_unaligned"]'
  -F context= \
  -F source_code= \

Note that the object contains 3 functions, but the final scratch is properly filtered down to just __copy_longs_unaligned, because of the value passed for diff_flags.

Resolves #215