feat: move car program to a separate module

That way, this repo doesn't end up bringing in urfav/cli as a dependency.

car/go.mod

+module github.com/ipld/go-car/car
+
+go 1.13
+
+require (
+	github.com/ipld/go-car v0.0.5
+	github.com/urfave/cli v1.22.4
+)
+
+// Only applies when built by checking out the source directory.
+replace github.com/ipld/go-car => ./..

go.mod

@@ -10,7 +10,6 @@ require (
 	github.com/ipld/go-ipld-prime-proto v0.0.0-20191113031812-e32bd156a1e5
 	github.com/multiformats/go-multihash v0.0.5
 	github.com/stretchr/testify v1.3.0
-	github.com/urfave/cli v1.20.0
 )
 
 go 1.13

go.sum

@@ -331,8 +331,6 @@ github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+
 github.com/stretchr/testify v1.3.0 h1:TivCn/peBQ7UY8ooIcPgZFpTNSz0Q2U6UrFlUfqbe0Q=
 github.com/stretchr/testify v1.3.0/go.mod h1:M5WIy9Dh21IEIfnGCwXGc5bZfKNJtfHm1UVUgZn+9EI=
 github.com/syndtr/goleveldb v1.0.0/go.mod h1:ZVVdQEZoIme9iO1Ch2Jdy24qqXrMMOU6lpPAyBWyWuQ=
-github.com/urfave/cli v1.20.0 h1:fDqGv3UG/4jbVl/QkFwEdddtEDjh/5Ov6X+0B/3bPaw=
-github.com/urfave/cli v1.20.0/go.mod h1:70zkFmudgCuE/ngEzBv17Jvp/497gISqfk5gWijbERA=
 github.com/warpfork/go-wish v0.0.0-20180510122957-5ad1f5abf436 h1:qOpVTI+BrstcjTZLm2Yz/3sOnqkzj3FQoh0g+E5s3Gc=
 github.com/warpfork/go-wish v0.0.0-20180510122957-5ad1f5abf436/go.mod h1:x6AKhvSSexNrVSrViXSHUEbICjmGXhtgABaHIySUSGw=
 github.com/warpfork/go-wish v0.0.0-20190328234359-8b3e70f8e830 h1:8kxMKmKzXXL4Ru1nyhvdms/JjWt+3YLpvRb/bAjO/y0=