Add control API VP9E_SET_TARGET_LEVEL that allows the encoder to
control the output bitstream level and/or keep level related
statistics.

Usage:
               255         do not care about level (default)
               0           keep level related stats only
               10          target for level 1
               11          target for level 1.1
               .
               .
               .
               62          target for level 6.2

Usage for vpxenc:

--target-level=0/255/10/11...

Change-Id: I31d1aeca19358b893e7577b4e63748c8e614034a