Make readme checking case- and extension-insensitive

[nnmrts]

Is your feature request related to a problem? If so, please describe.
My (and many other projects) don't have a readme file in their repo that's exactly called README.md. Currently the readme filename has to be exactly this, otherwise the module page will say "No README found for this module". A lot of projects have a readme.txt or README.txt, or just README. I've also seen readme.md a lot and use it myself.

Describe the solution you'd like
Just relax the readme check a bit. GitHub for example accepts the above mentioned variations as well.

Describe alternatives you've considered
A config field in egg.json, like readme, where one could set this to a relative path. Personally I would prefer nest.land doing it automatically though.

[UltiRequiem]

Please!