md_eval.pl -- Provide evaluable markdown

This module adds evaluable sections to markdown cells in SWISH notebooks. Such cells are written as

```{eval}
<Prolog code>
```

 swish_provides(?Term) is nondet

True when Term describes a provided feature of the current SWISH instances. Provided Term values are:

plugin(Name)

True when Name is the name of a loaded plugin

In addition, plugins may provide additional terms by adding facts to swish_config:config(provides, Term).