It would be nice if variables could be used in the README.md

[justinmchase]

In most / all readme's there is usually a little usage example to get someone started. It will usually start with a line that shows how to import the current module. That import statement usually should have a version number in it. Trying to keep that version number up to date with the actual latest version tag isn't super hard but it does require a little automation, or dilligence, or both.

What would be nice is if deno.land/x recognized, at least, the $GIT_TAG variable and just did a straight replacement, so if I had some snippet in my readme such as: like:

```ts
import { example } from "https://deno.land/x/example@$GIT_TAG/mod.ts";
```

Then it would render like this, when viewed relative to the tag named 0.1.0 in the deno land third party module UI.

import { example } from "https://deno.land/x/example@0.1.0/mod.ts";

[kitsonk]

This is a suggestion for dotland: denoland/dotland#2494