replace WaitGroup with errgroup to allow error handling and propagation

The program now exits with a non-zero code when errors are encountered
while reading and/or writing files.

This now also introduces Go Modules for dependency management

.travis.yml

@@ -1,13 +1,13 @@
-dist: xenial
+dist: bionic
 git:
   depth: 3
 language: go
 go:
-  - "1.x"
-  - "1.10.x"
+  - 1.13.x
 before_script:
   - go get golang.org/x/lint/golint
 script:
+  - go mod tidy
   - gofmt -d -e -l -s .
   - golint -set_exit_status ./...
   - go test -v ./...