Default aliases

[lencioni]

I've been thinking about how we can make import-js more useful for folks out of the box, without any configuration.

Besides defaulting to relative paths (#186), I think it might be interesting to include some default aliases that are only activated when you have the aliased package in your dependencies, and can be overridden by configuration. This would work well for a few packages that are commonly aliased, like $ for jquery and _ for underscore or lodash. There may be others that we could add as we go along.

[trotzig]

I like this idea!

[lencioni]

I wonder if this should be combined with #107 and just allow this to be defined in the project's package.json.