Upgrade Earmark to v1.4.15

Fixes various Markdown-related issues.

Earmark has changed a lot, and it seems the custom renderer added in !2204 (merged) is no longer supported. Its only purpose was to strip \n from the output, so I removed the custom renderer and just stripped \n with String.replace/3 instead.

Running the renderer tests on Utils.format_input/2 revealed a lot of problems with the parser that exist even in the current develop branch. This could be merged as-is, but I'm gonna try to fix those first.