Adds porcelain option to droplets command

[petems]

• Easier output for things like sed, awk and cut
• Example from tugboat droplets | grep -F TEST | cut -d : -f 2 | cut -d ' ' -f 2 | cut -d , -f 1 >>big-test-hosts
• This would now be accomplished with something as easy as:

  bundle exec tugboat droplets --attribute=ip4
  test-blog-1,41.101.11.22
  test-blog-2,138.68.11.22

[coveralls]

Coverage decreased (-0.1%) to 98.0% when pulling 799a727837bfa86c551cbcb0a8766f21eb02918d on add_porcelain_and_attribute_option_to_droplet_list into a4c4c9cfed4f1d2f3fd8c2afb3a4da466a527052 on master.

[coveralls]

Coverage increased (+0.05%) to 98.175% when pulling b646b51b90cebecf73d4fb1e50819646cbe3c90b on add_porcelain_and_attribute_option_to_droplet_list into a4c4c9cfed4f1d2f3fd8c2afb3a4da466a527052 on master.

[coveralls]

Coverage increased (+0.05%) to 98.175% when pulling b646b51b90cebecf73d4fb1e50819646cbe3c90b on add_porcelain_and_attribute_option_to_droplet_list into a4c4c9cfed4f1d2f3fd8c2afb3a4da466a527052 on master.

[coveralls]

Coverage increased (+0.05%) to 98.175% when pulling b646b51b90cebecf73d4fb1e50819646cbe3c90b on add_porcelain_and_attribute_option_to_droplet_list into a4c4c9cfed4f1d2f3fd8c2afb3a4da466a527052 on master.