Remove necessity to append .md to directories

[klawr]

The performance boost shouldn't be to big on normal projects like

• https://goessner.github.io/microjam/
• https://goessner.github.io/g2/
• https://goessner.github.io/Mechanismentechnik/

and I do think it is confusing to add file endings to directories.

A better approach may be to add a blacklist (or whitelist?) to the microjam object in the package.json, or to check directories for a microjam.ignore file.