Fix #34: Relative path support improved by using base tag

[yhatt]

I improved relative path support by using <base> html tag.

v0.0.8 can not resolve relative path except markdown's image format (![]()). But now <base> tag's href attribute can set specific starting directory for resolve. It means relative path would be supported in <img> tag and your custom styling.

This affect Marp resource's path (CSS, Throbber). So I made path resolver for these.

[yhatt]

Oops. The image path in the last slide of Gaia theme example is wrong.

[yhatt]

I fixed the example. However I found a new bug of absolute path.

The path splitter \ in Windows makes the path conversion fail in only Markdown format ![]() only. (¥ means same in Japanese Windows, and its rendering is reported already at #46)

I'll fix its counterintuitive behavior because of it affects to the feature of quick image insertion by D&D.

[yhatt]

OK. a4a69dc fixed the problem of absolute path by reverting previous custom renderer. And it still has the improved path resolver.

I would merge it if we have not seen the other problems.