readme: fix a mistake
npm: 0.0.67 -> 0.0.68
Merge branch 'release/0.0.68' into develop
src: add instance method 'rightCols()'

fix a mistake
src: add instance method 'middleCols()'
test: add instance method 'middleCols()' testings

fix some mistakes
lint: add some files for linting pass
readme: add instance method 'middleCols()' api