Officially support more elaborate externals

Bắt đầu với việc xử lý external và exclude trong src/index.js#L387-L388, sau đó xem xét hành vi dòng lệnh được đề xuất trong issue cho --external và --external-glob. Công việc được xem là hoàn tất khi ngữ nghĩa cấu hình được chọn đã được triển khai và ghi lại trong tài liệu, bao gồm cả cách các dependency và import không theo chuẩn được xử lý.

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.