feat: add app stats output

[ctrox]

This adds a new stats output format for the get app command. It prints status information about the app replicas and also some very basic metrics. The get app help has been extended to explain all the columns of the stats output.

Here's what the output looks like:

$ nctl get apps -o stats
PROJECT        NAME    REPLICA                  STATUS   CPU   CPU%   MEMORY   MEMORY%   RESTARTS   LASTEXITCODE
nine-staging   rails   rails-9f6bdb676-crlrf    ready    1m    0.8%   121MiB   47.4%     0          <none>
nine-staging   rails   rails-9f6bdb676-2pfps    ready    1m    0.8%   123MiB   48.4%     0          <none>
nine-staging   rails   rails-9f6bdb676-sd9mm    ready    1m    0.8%   122MiB   48.0%     1          137 (Out of memory)