Serving for a directory support.

[simonmysun]

Hi I have added the feature I requested yesterday(#14), with some tricky ways. You can run markmon ./ to view a directory of markdown files.

Now I can write hyperlinks in the markdown file and just click it to view what it links to. That's really helpful when I'm dealing with large articles. I split it into parts, and edit them respectively.

However there's some problem:

• The original hash tag is replaced, so the anchors may cannot be automatically jump to. But a little additional JavaScript can fix this;
• You have to make two request to open a page, which makes it a bit slow;
• I haven't edited the document. I don't know how;
• I don't think it will work fine with large amount of requests and file changes.

If you are satisfied to my pull request you can merge it.