Allow consul exec output to be machine readable

[levenaux]

When programmatically executing consul exec we currently have to parse the output to get the values we need.
It would be nice to have a way of executing the consul exec with -machine-readable to stop any verbose output coming from Consul.

[slackpad]

Might be nice to have something like a -json output that provides a structured representation of the results, which would let you use jq or similar to pick out the bits you need.

[imcom]

this could be a really killer-feature! BTW, for Consul exec, is there an intention to make it available through an API?