Install Arcanist

Summary:
This script helps to bootstrap Arcanist by cloning the right repositories.

Expected to work on FreeBSD, Linux, macOS and Cygwin.

Test Plan: sudo ./utils/install-arc.sh

Reviewers: Sandlayth, DorianWinty

Reviewed By: DorianWinty

Subscribers: MoustaphaAs

Differential Revision: https://devcentral.nasqueron.org/D3434