Create macOS installer

[cosmo0920]

macOS installer should be provided as dmg.
And macOS version of bundled Ruby should openssl reference.
So, we should bundle OpenSSL library for macOS.

• Create dmg installer
• Create Homebrew td-agent Formula
• Register dmg installer into Homebrew cask

[cosmo0920]

Creating dmg format installer is done by #192.

[cosmo0920]

Creating Homebrew td-agent Formula task is also treated in this issue.

[cosmo0920]

Registering td-agent dmg installer into Homebrew Cask repository is also needed to handle in this issue.