Wrap README.md to 80 columns
Wrap INSTALL to 80 columns
Wrap CONTRIBUTING.md to 80 columns
Merge pull request #2595 from drbrain/80_column_README

Wrap README, etc. to 80 columns
Fix `mrb_get_args` arguments types found by mruby-clang-plugin.
Merge pull request #2596 from take-cheeze/fix_args

Fix `mrb_get_args` arguments type error found by mruby-clang-plugin.