Provide an option to disable color in `droplets` command

[larsks]

While the colorized output of tugboat droplets is nice when visually inspecting the output, it can cause problems when one wants to use the command in scripts (either logging information to a file, or using it to check the status of a droplet).

An option to disable color would be helpful, as would making tugboat behave more like other software that automatically disables color codes when stdout is not attached to a tty.

[petems]

Lets see how easy this would be: http://stackoverflow.com/questions/33305188/disable-color-for-thor