Extract abstract template engine out.

[ryan-roemer]

We have a generic templating tool here that's not builder specific, so let's move that to it's own project

• Move all of lib/ to new project.
• Move the bin/builder-init.js logic to a lib/ library and leave a very small shell.
• Take default prompts values as an argument to executor in new project.
• Proposal: Make all tool-provided automatic values prefaced with an _. So, e.g., instead of eslint, npmignore, etc. we'd have _eslint, _npmignore, etc. in new tool
  • Deprecate eslint, npmignore, etc. in builder-init and provide transition aliases and documentation.
• Open Question: Make destination into _destination and just another normal field that if empty blows up?

New list of proposed default fields:

/**
 * Default prompts values added to all prompt calls.
 */
var DEFAULTS = {
  destination: {
    message: "Destination directory to write",
    validate: function (val) { /* existing stuff */ }
  },
  derived: {
    // Existing renamed file name helpers.
    _npmignore: function (data, cb) { cb(null, ".npmignore"); },
    _gitignore: function (data, cb) { cb(null, ".gitignore"); },
    _eslintrc: function (data, cb) { cb(null, ".eslintrc"); },
    _npmrc: function (data, cb) { cb(null, ".npmrc"); }

    // New stuff
    //
    // Location of templates directory to use.
    // Default to `init` for `builder-init`. Could be anything for new project.
    // https://github.com/FormidableLabs/builder-init/issues/36
    _templatesDir: /* snipped */
  }
};

/cc @kenwheeler