Changes around shell variable handling, exporting databases or cypher results
and general shell improvements

o no welcome message with command mode (-c), also configurable with -Dquiet=true
o shell variables passed to cypher queries as parameters
o last cypher result row content set as shell variables
o allow json for shell variable values, maps, arrays, etc
o dump command for exporting a db or the subgraph of a cypher statement
o can reassign shell variables, export a=10 && export b=a -> b=10
o shell can execute a contents file directly with -f file which is faster
  than piping via stdout
o changed SubGraph to be an interface and, only wrap the DB for DatabaseSubGraph