Add bleach

for sanitizing HTML. Example:

```js
languagePluginLoader.then(() => {
  // pyodide is now ready to use...
  pyodide.loadPackage('bleach').then(() => {
    pyodide.runPython(`
import bleach

print(bleach.clean('There is good stuff <script>and bad stuff</script> in here.'))
    `);
  });
});
```

leads to the output:

```
There is good stuff &lt;script&gt;and bad stuff&lt;/script&gt; in here.
```

packages/bleach/meta.yaml

@@ -0,0 +1,13 @@
+package:
+  name: bleach
+  version: 3.1.0
+source:
+  sha256: 3fdf7f77adcf649c9911387df51254b813185e32b2c6619f690b593a617e19fa
+  url: https://files.pythonhosted.org/packages/78/5a/0df03e8735cd9c75167528299c738702437589b9c71a849489d00ffa82e8/bleach-3.1.0.tar.gz
+requirements:
+  run:
+  - setuptools
+  - webencodings
+test:
+  imports:
+  - bleach

packages/webencodings/meta.yaml

@@ -0,0 +1,9 @@
+package:
+  name: webencodings
+  version: 0.5.1
+source:
+  sha256: b36a1c245f2d304965eb4e0a82848379241dc04b865afcc4aab16748587e1923
+  url: https://files.pythonhosted.org/packages/0b/02/ae6ceac1baeda530866a85075641cec12989bd8d31af6d5ab4a3e8c92f47/webencodings-0.5.1.tar.gz
+test:
+  imports:
+  - webencodings