Enable mapping of parameters/variables from a file

[simonsabin]

As an extension of #386 it would be nice if you could pass variables similar to a tfvars file in terraform to sqlcmd.

i.e. if I specify a JSON file or CSV file the values are mapped to the variables/parameters

given a json file of variables.json

{
 "Somevar":"a value",
 "Anothervar":"another variable"
}

Running sqlcmd -F variables.json would be equivalent of sqlcmd -v Somevar="a value" -v Anothervar="anothervariable"

This avoid having to build complex command executions