PublicShow -- Provide evaluable markdown

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

<Prolog code>
Source swish_provides(?Term) is nondet
True when Term describes a provided feature of the current SWISH instances. Provided Term values are:
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).