add minimal configtest command

[josephholsten]

Minimalist implementation of #901. Needs:

• some help text
• better error handling
• test cases

Open questions:

• should this be named consul configtest or some other color of bike shed? This is similar to apachectl configtest.
• are there any other use cases that should be included in this? We can always add more later.

[ryanuber]

Can we add a more verbose description? It would be helpful to explain what exactly the command does and why it is useful.

[josephholsten]

How's this sound?

Tests that configs are valid by attempting to parse them. Useful to ensure a configuration change will not cause consul to fail after a restart.

[ryanuber]

@josephholsten this looks much better, thanks! RE: command naming, I think configtest is fine. I left some minor comments, and it looks like we are missing the middleman documentation, so if you want to add that feel free. Otherwise, we can also take care of it after a merge.

[josephholsten]

I'll squash commits once we're ready.

[josephholsten]

@ryanuber care to review the code again? I'll get the middleman doc ready

[josephholsten]

Should be fully tested, doc'd, squashed and ready to roll.

[ryanuber]

This description needs some updating

[josephholsten]

ha! updated.

[ryanuber]

This LGTM. Thanks!