Dynamic prop/attr names

Not the same as #109.

That would be useful to have insertable attribute names:
```js
html``
// same as
html``
```

Optional attributes:
```js
html``
```

let _data = Symbol('data')
html``

Anonymous props:
```js
html``

@developit @jviide would you consider that feature?

No file or test is named in the issue; start by reading issue #109 alongside the examples here to define the intended distinction. Use the optional, private, and anonymous prop examples as acceptance cases; done means the feature supports these forms without conflating them with #109.

索引モデルが issue の本文から書いたものです。