Use the textContent of pyxel-run tag instead of script attribute
Hi, I read the docs on https://github.com/kitao/pyxel/wiki/How-To-Use-Pyxel-Web#write-an-html-file-with-pyxel-custom-tags.

The mentioned code snippets maybe improve to the following code just like script or style tag.

```html
<pyxel-run script="
import pyxel
pyxel.init(200, 150)
pyxel.cls(8)
pyxel.line(20, 20, 180, 130, 7)
pyxel.show()
"></pyxel-run>
```

```html
<pyxel-run>
import pyxel
pyxel.init(200, 150)
pyxel.cls(8)
pyxel.line(20, 20, 180, 130, 7)
pyxel.show()
</pyxel-run>
```