Support external ref list, support more platform.

Fixes

• fix: when output-dir is not asset-output-dir, it generates wrong url.

• fix: use the title to generate ref list text, instead of using link id. Make the ref-list more human friendly.

• fix: output ref-list and ref-def for only used links. Unused links will not appears on the bottom.

Features

• new-feature: add platform minimal_mistake: to convert md to minimal_mistake compatible version

• new-feature: add --rewrite to rewrite generate url, when one need to customize urls before uploading assets.

• new-feature: add arg "--jekyll" to respect jekyll syntax markdowns.

• new-feature: add option --refs to add external reference definition

Doc

• doc: fix readme: update according to latest changes