add coveralls badge

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 [![GoDoc](https://godoc.org/github.com/alexflint/go-arg?status.svg)](https://godoc.org/github.com/alexflint/go-arg)
 [![Build Status](https://travis-ci.org/alexflint/go-arg.svg?branch=master)](https://travis-ci.org/alexflint/go-arg)
+[![Coverage Status](https://coveralls.io/repos/alexflint/go-arg/badge.svg?branch=master&service=github)](https://coveralls.io/github/alexflint/go-arg?branch=master)
 
 ## Structured argument parsing for Go