Enable loading of .vue files from npm packages

[akanix42]

There are quite a few Vue packages on npm where the author doesn't provide a compiled version. This change allows users to load .vue files from npm packages, e.g. import Dropdown from 'vue-my-dropdown';.
Without this, attempting to import a .vue file from an npm package causes the following error:

C:\projects\oss\tests\vue-component-test\node_modules\vue-my-dropdown\src\vue-my-dropdown.vue:1
(function (exports, require, module, __filename, __dirname) { <template>
                                                              ^
SyntaxError: Unexpected token <
    at exports.runInThisContext (vm.js:53:16)
    at Module._compile (module.js:373:25)
    at Object.Module._extensions..js (module.js:416:10)
    at Module.load (module.js:343:32)
    at Function.Module._load (module.js:300:12)
    at Module.require (module.js:353:17)
    at require (internal/module.js:12:17)
    at npmRequire (C:\projects\oss\tests\vue-component-test\.meteor\local\build\programs\server\npm-require.js:133:10)
    at Module.useNode (packages\modules-runtime.js:687:18)
    at fileEvaluate (packages\modules-runtime.js:332:20)

The second commit fixes an issue with esModule detection (Meteor modules use a esModule symbol but most transpiled modules are looking for a simple __esModule property). In combination with the above, this enables the use of packages such as vue-progressbar (#177).

An example project (not including the vue-component) can be found here: https://github.com/nathantreid/vue-component-test/tree/load-vue-component-from-npm.
If you're good with this change I can also update the npm section of the vue-component documentation.

[Akryum]

Awesome PR! 😄