impelement --code-file and --file for parity

[mikedanese]

I was thinking that it would be nice to write a unit test runner that was used like this

$ jsonnet --code-file cases=test_suite/slices.jsonnet stdlib/tests/runner.jsonnet

Prereq is --code-file. @sparkprime I think we talked about this.

$ cat in.jsonnet 
local a = 1;
local b = 2;

a + b
$ jsonnet --code-file cases=in.jsonnet --exec '{cases: std.extVar("cases")}'                                                               
{
   "cases": 3
}
$ jsonnet --file cases=in.jsonnet --exec '{cases: std.extVar("cases")}'                                                                    
{
   "cases": "local a = 1;\nlocal b = 2;\n\na + b\n"
}