+fix(git): commit-msg hook didn't work on mac
+fix(git): commit-msg hook didn't work on mac

Sed on mac uses the BSD flavor instead of the GNU one. gnu-sed can be
installed through homebrew and used through `gsed` instead of `sed`.

Modify the script so that it handles this issue by detecting the OS.

franciscoj/dot-files,franciscoj/dot-files,franciscoj/dot-files