thoughts on extensible custom tags

So this may not be in the philosophy of bel, but how/should/can I easily create an extensible module for bel/yoyo (the module I use) that allows for custom tags/props that just render functions.

Here is what I'm thinking:

```
const Button = function (opts) {
return bel`my ${opts.name} button`
}

const Col = function (_yield, opts) {
return bel`

`
}

const customtagsuse = bel`

`

// which is actually just

const customtagsuse = bel`

`

// or

const customtagsuse2 = bel`

`

// which is actually

const customtagsuse2 = bel`

`
```

Kinda reactish, but all it's doing is regexing the string, and replacing it with the element.

I'm thinking of providing a layer to bel.

so something like:

```
var extensible = require("bel-custom")
var bel = extensible(require("bel))
bel.customElements = {"Button": Button}; // sort of thing
```

Any thoughts on design patterns, approaches, potential pitfalls, potential benefits @shama?