Rename STR_BUF_MIN_SIZE to MRB_STR_BUF_MIN_SIZE. Make it configurable.
Merge pull request #982 from monaka/pr-make-MRB_STR_BUF_MIN_SIZE-configurable

Rename STR_BUF_MIN_SIZE to MRB_STR_BUF_MIN_SIZE. Make it configurable.