Add support for running kubectl plugins

[everettraven]

While buoy is a tui client for kubernetes, there are many helpful plugins that already exist for kubectl. buoy should support some form of running a kubectl plugin and presenting the output in a dashboard panel. Since buoy is a dynamic dashboard, when specifying a kubectl plugin to run a user should also specify how frequently that plugin should be re-run to refresh the output - buoy should not make any assumptions here.