Add Custom Export Map resolution to esinstall

[ryansolid]

Rollup Plugin Node Resolve 11.0.0 adds support for custom export map resolution:
https://github.com/rollup/plugins/tree/master/packages/node-resolve#exportconditions

This is important for universal package aliasing for non-node environments.

[FredKSchott]

+1 for this, I'd spin off a prioritized issue on the issue tracker but looks like you already created a PR :) will continue the convo there