Add API alias options

[legendecas]

Exposing alias options allowing to map one module to another and bundle the mapped module into the final archive.

Rollup alternatives: https://github.com/rollup/plugins/tree/master/packages/alias

Related: #460

[styfle]

Suggested change

	Object.keys(alias).forEach(from => aliasMap[from] = alias[from]);
	aliasMap = alias;

I think you can avoid iteration here an assign instead

[styfle]

What is the use case for this? Should there be a CLI option too?

[morgs32]

I'm interested in this - wondering if it would enable transpiling other packages in a yarn workspace/monorepo? For example:

// packages/package1.js

import utils form 'package2'

If the main in package2's package.json is es6 or typescript, could it still be transpiled if we alias'd to it with a relative path instead of referencing the same package via node_modules in a yarn workspace (which points to the directory via symlink)