Migrate from Dep to Go Modules

Go Modules are the preferred way to manage dependencies in Go programs
since version 1.11. The migration process involves:

 1. go mod init git.dominic-ricottone.com/~dricottone/image2ascii
    (replace the host as needed)
 2. Replace the host in files requiring an internal module
    (this was specifically image2ascii.go, convert/convert.go,
    convert/resize.go, and convert/resize_test.go)
 3. go mod tidy
 4. rm -rf vendor/ Gopkg.lock Gopkg.toml
 5. go get -u

go.mod now contains all the information required to build the program.