Shell import export

Changes around shell variable handling, exporting databases or cypher results
and general shell improvements

* no welcome message with command mode (-c), also configurable with -Dquiet=true
* shell variables passed to cypher queries as parameters
* allow json for shell variable values, maps, arrays, etc
* dump command for exporting a db or the subgraph of a cypher statement
* can reassign shell variables, export a=10 && export b=a -> b=10
* shell can execute a contents file directly with -f file which is faster
  than piping via stdout