Adding new sub-command "field" to "droplet"

To make scripting of pontoon commands (such as chaining them in bash scripts)
a lot easier, a new sub-command was added to the DropletCommand that allows
querying/outputting the value of a single field of a droplet's meta-information.

I use it in some helper bash functions like [this](https://github.com/nureineide/dotfiles/commit/42257cb1109e00504a5b001a9f3462d0194ea038#diff-1)