Improve import normalization

[migerh]

Currently the import normalization in lib/parseImports.js appends '.js' or '.jsx' and disregards any filename extensions that might already be specified. With module bundlers (e.g. webpack or Meteor's build system) also allowing to import static assets like css or text this needs to be improved.

• Check the standard what filename extensions may be omitted, use that as a default
• Add a configuration option, maybe something like Webpack's resolve.extensions config (See [1])

[1] https://webpack.github.io/docs/configuration.html#resolve-extensions