Create a formula for singularity

[sjackman]

Please create a formula for Singularity and submit it to https://github.com/Linuxbrew/homebrew-extra http://singularity.lbl.gov/quickstart

brew create --tap=linuxbrew/extra https://github.com/singularityware/singularity/releases/download/2.3.1/singularity-2.3.1.tar.gz