Add support for local links(change file serving so server based)

[ShyykoSerhiy]

[boneskull]

indeed, I'm unable to show local images

[josdejong]

This would be very useful!

Indeed images are not working in Gfm Preview, whilst they work fine in HTML Preview and Preview.

[josdejong]

btw thanks for this great plugin @ShyykoSerhiy, this is the first decent markdown plugin for IntelliJ!

[EvgenyVetrov]

Realy i cant go to local link.

I have 2 files: docs/readme.md
docs/about.md

Test link to about.md [Это пробная ссылка на соседний файл ./about.md - этот.](./about.md)

And this link dont work %-(