Enable to set multiple values with the same key

[qmachard]

This PR is only a Typing changes to support multiple values with the same key

const templ = uriTemplate.parse("http://localhost:8080/api/contacts{?status}");
const expanded = templ.expand({status: ["valid", "waiting"]});

// http://localhost:8080/api/contacts?status=valid,waiting

Resolves #421

[evert]

Lets do it!

[remidewitte]

Hello,
Any chance to have this change on a 7.x release ?

Many thanks !
Rémi