Make str_modify public
Merge pull request #1188 from h2so5/make-str_modify-public

Make str_modify public
Add String#swapcase,swapcase!,concat,casecmp,start_with,end_with
Merge pull request #1189 from h2so5/mruby-string-ext-addition

Add mruby-string-ext methods
Merge branch 'master' into iij