Add Homebrew formula

[ThatIsAPseudo]

As pointed in #3, there should be a Homebrew [1][2] formula to make installation on macOS easier. Here [3] [4] is the ledger formula and here [5] is explained how to cook a formula (I'll try to see if I can do it).

[1] https://brew.sh/ [2] https://github.com/Homebrew/homebrew-core [3] https://github.com/Homebrew/homebrew-core/blob/HEAD/Formula/ledger.rb [4] https://formulae.brew.sh/formula/ledger [5] https://docs.brew.sh/Adding-Software-to-Homebrew