Officially support more elaborate externals

Comienza con el manejo de external y exclude en src/index.js#L387-L388 y, después, revisa el comportamiento propuesto en el issue para --external y --external-glob en la línea de comandos. Se considera terminado cuando la semántica de configuración elegida esté implementada y documentada, incluido cómo se tratan las dependencias y los imports no estándar.

This is a feature request/Proposal

As a user of microbundle I want ways to configure which imports are to be considered external.
In order to reduce my bundle size and also to work with non-standard imports (images, fonts, ...) that are not supported by microbundle.

Status Quo

1. As described in https://github.com/developit/microbundle/issues/283#issuecomment-523879163, there is a (I guess unoffical) way to use regex in the --exclude flag, which can be used to have rollup treat specific import statements as external.
2. Presence of the --exclude flag triggers if pkg.dependencies are treated as external or not (see index.js#L387) I find that quite confusing and the behaviour is not documented.

Problem

Given I want to use --external .*/assets/.* to work with non-standard imports
And I also want all my dependencies to be excluded
Then I need to add all dependencies to --external causing code duplication and manual overhead.

Proposal

In order to fix the problem and reduce confusion I propose to have it work like this:

$ microbundle

Exclude dependencies, peerDependencies

$ microbundle --external .*/assets/.*

Exclude dependencies, peerDependencies and .*/assets/.*
And explicitly support regular expressions or globs under --external

$ microbundle --external .*/assets/.*,angular --include-dependencies

Exclude peerDependencies and .*/assets/.*
All normal dependencies ecxept angular are included in the bundle

Non-breaking alternative

I realize that the above proposal is really tied to my personal usecase of (miss-?)using --external for non-standard imports.
It would also introduce a potential pretty impactful breaking change (since users of microbundle might ship bundles that lack bundled dependencies).

Alternatively the current behaviour could stay as is, using regex in --external will be considered hacky. And we introduce a new, documented way to exclude assets:

$ microbundle --external-glob **/*.(jpg|png)

Exclude dependencies, peerDependencies and all imports matching **/*.(jpg|png)

$ microbundle --external angular --external-glob **/*.(jpg|png)

Exclude peerDependencies, angular, and all imports matching **/*.(jpg|png)

Let me know what you think about this. I would be really happy to implement the changes and provide a PR.