Support query templates

[robertmuil]

Define a syntax and format (.rqt) that allows variable placeholders, such that the query command can give a template and variable bindings on the CLI.

e.g.:
pythinfer query 'SELECT ?s { ?s rdfs:label "?[NAME]" }' 'Joe Bloggs'

Options for placeholder syntax:

• double curly braces: {{ ... }} - looks like SPARQL, easily confused
• query and curly braces: ?{{ ... }} -
• double square braces: [[ ... ]] + similar to natural way to add placeholder in text, - easily confused with blank nodes
• query and single square braces: ?[ ... ] + very similar to natural placeholders, and variable syntax, +not valid sparql syntax
  ...