Add tap-configure cmd

Basically to override the default ruby path and root directory.

CmdUtils.CreateCommand({
  name: "tap-config",
  homepage: "http://tap.rubyforge.org",
  author: { name: "Simon Chiang", email: "simon.a.chiang@gmail.com"},
  license: "MIT",
  description: "Sets a Tap configuration.",
  takes: {"key": noun_arb_text},
  modifiers: {"value": noun_arb_text},

  execute: function(key, mods) {
    // preview current values
    // override
    // error for unknown key
    // reset default if value is default
  }
})