Swapped syntax for escapable and literal text.
 Add a filter that manipulates the YAML node graph
 produced by yaml.compose and replaces the tags
 so that numbers, nulls, booleans and so on
 are not converted to the Python type but
 kept as text.

 Also added some prototypical template support
 to the filter: the filter will not modify
 nodes tagged '!stubbly', except for nodes
 (or child nodes) tagged '!stubbly/quote-as-strings'.