add interactive options to `start` to allow viewing and setting config

[sethbattin]

The idea here is that when you start with a certain option, e.g. gluestick start -i , you get an interactive prompt rather than an immediate execution. This should let you change config parameters if you want, but also see the parameter's current value and elect to use them or override them.

/cfe/$ gluestick start -i
starting gluestick app.  [return] to accept current settings
API_HOST (https://qa.api.myapp.com/) ?
NODE_ENV (development) ?
config_thing (2) ? 17
executing
  API_HOST="https://qa.api.myapp.com/" \
  NODE_ENV=development \
  config_thing=17 \
  gluestick start

so that -^ with the option to change those values if needed.

The ENV stuff is somewhat straightforward (I hope?), but the config stuff might be more complicated.

Obviously there's no way for gluestick to know what all is necessary and what isn't, parsing process.env would be especially ridiculous. So that implies some manner of configuration for a project to specify what settings are interact-able.