Implement caching?

[fwolfst]

I just stumbled around https://www.lugolabs.com/articles/render-markdown-views-with-redcarpet-and-pygment-in-rails . I like the class setup (#erb with memoization, #markdown, #render and #call, but imho should add #redcarpet_options) and he or she is adding caching functionality at the very and of the article. Sounds like a very low hanging fruit.

I havent done any caching research, in able to render a markdown file with different renderers (e.g. TOC), I guess the cache key should not be generated only from the file content, but also from another value like :emd_redcarped_md` or something like that, but really just guessing here.

I am a bit pessimistic that I will find time to craft a PR.

Are we up to emd 0.2.0? :)