Support for "incremental" mode

[aleclarson]

This plugin should respect "incremental": true in the local tsconfig.json module.

Also, tsBuildInfoFile should be supported. 👍

[Swatinem]

Since I’m experimenting with project references and incremental right now myself. I thought about:

just use tsc to compile .ts -> .js / .d.ts, and then use rollup to bundle up .js without any transforms, and rollup-plugin-dts to bundle up .d.ts.

Haven’t had too much time lately to work on it though.

[Swatinem]

This will not be implemented as per #29
you can run tsc with --incremental and declaration: true / emitDeclarationOnly: true and then use the output files with this plugin