Support custom extension for url

[adrianEffe]

for instance by running:

curl --verbose \
  --header "Content-Type: application/json" \
  --request POST \
  --data '{"url":"https://www.rust-lang.net","extension":"rust"}' \
  http://127.0.0.1:8000/shorten

and then calling:

curl --verbose \
  --header "Content-Type: application/json" \
  "http://127.0.0.1:8000/rust"

should redirect to https://www.rust-lang.net