Support URLs as templates

[joerdav]

A convenience feature could be to use http urls to reference templates:

mkdir generated
oto -template https://raw.githubusercontent.com/pacedotdev/oto/master/otohttp/templates/server.go.plush \
    -out ./generated/oto.gen.go \
    -ignore Ignorer \
    -pkg generated \
    ./definitions
gofmt -w ./generated/oto.gen.go ./generated/oto.gen.go
oto -template https://raw.githubusercontent.com/pacedotdev/oto/master/otohttp/templates/client.js.plush \
    -out ./generated/oto.gen.js \
    -ignore Ignorer \
    ./definitions
    ```