Add --compact, -c bool flag to omit nonessential whitespace

[zmedico]

Also submitted upstream as jmespath#29, and to go-jsonfile#1.

This version does not include the short -c flag, since that was taken by the --color flag.

[zmedico]

I've created an extended version of jp called jpp which now has the --compact option:

pipebus/jpp@ee70420

NAME:
   jpp - jpp [<options>] <expression>

USAGE:
   jpp [global options] command [command options] [arguments...]

VERSION:
   0.1.3.1

COMMANDS:
   help, h	Shows a list of commands or help for one command
   
GLOBAL OPTIONS:
   --accumulate, -a	Accumulate all output objects into a single recursively merged output object.
   --compact, -c	Produce compact JSON output that omits nonessential whitespace.
   --filename, -f 	Read input JSON from a file instead of stdin.
   --expr-file, -e 	Read JMESPath expression from the specified file.
   --slurp, -s		Read one or more input JSON objects into an array and apply the JMESPath expression to the resulting array.
   --unquoted, -u	If the final result is a string, it will be printed without quotes.
   --ast		Only print the AST of the parsed expression.  Do not rely on this output, only useful for debugging purposes.
   --help, -h		show help
   --version, -v	print the version