feat: Add full-text content output

[Maecenas]

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Add full-text content output.

Description

Add as a default option to export full-text content.

Checklist

• [x] All tests are passing
• [x] My code follows the code style and structure of this project

You may preview the sample feed output at my blog. One thing to notice is that I haven't exported the canonical src for <img> tags. Local development with Reeder 4 would correctly display the result (<img src='/pic.jpg'></img>), while Reeder 3 wouldn't. The default heading anchor (#) from VuePress also didn't been removed from the output.

Development (Reeder 4) Development (Reeder 3)

[webmasterish]

Hi @Maecenas,

It needs to support any feed reader, that's why the paths need to be absolute.

[Maecenas]

Hi @webmasterish, The images on third-party CDN work there. The images with absolute src can be addressed by calling url.resolve(path: string): string. But for those images with relative src (./test.jpg, those placed in the same folder with markdown), I have no idea for now, as plugins like this are applied before the minification started.