Add support for memory limit.
Add support for memory limit.

This limit gets compared against the upper bound that's already provided
in README. Providing a facility to handle the memory limit internally
makes it easier to change the estimate used in the future.

Fixes #75.

ianhuang-777/guetzli,google/guetzli,szabadka/guetzli,szabadka/guetzli,guanzidashu/guetzli_ios_metal,dloeng/guetzli,guanzidashu/guetzli_ios_metal,google/guetzli,dloeng/guetzli,ianhuang-777/guetzli,google/guetzli,guanzidashu/guetzli_ios_metal,szabadka/guetzli,dloeng/guetzli,google/guetzli,guanzidashu/guetzli_ios_metal,ianhuang-777/guetzli,ianhuang-777/guetzli