RFC: Support slot attributes and data cascade

⚠️ **this feature is related to the [Collections proposal](https://github.com/giuseppeg/xm/issues/17) and probably should be implemented before #17**.

## slot attributes

Currently xm support only element slots:

```html

```

We should add support for slot attributes:

```html
contact me
```

These will be filled with fill elements eg:

```html
acme.com
```

### spec

Any attribute can be slotted if prefixed with `slot:`. Within the value for the attribute there can be multiple expressions delimited with `{` and `}`. The expression name matches a `fill` name, and xm replaces the expression with the `fill` value.

## Data Cascade

xm should allow to define multiple sources for data, including a global data object that is provided via config file.

We need to rework [the main plugin](https://github.com/giuseppeg/xm/tree/master/lib/posthtml-xm-import) to merge the data correctly using a Data Cascade mechanism similar to the 11ty's one. Check out https://www.11ty.dev/docs/data-cascade/

Additionally we need to provide a global `xm:permalink` fill value that contains the absolute path for the file so that people can use it to reference to the current page (eg. when creating sharing links). The `xm:` namespace will be reserved for xm fill values.