Add 500 GitHub issue middleware

This middleware created a new GitHub issue at every new 500 error response.

If the error has already been registered the associated GitHub issue get
updated by reopening it.

Note: error responses are registered using bodies as a key.
Add slugify function; update github config params

Note: slugify taken from leafo/lapis, see
https://github.com/leafo/lapis/blob/v1.0.6/lapis/util.lua#L108-L110
Add README
Refactor code to extract GitHub API requests from middleware

As suggested by @mikz this commit moves the code that makes requests to
GitHub API to helper functions. This update allows a faster implementation
of new middlewares with the same logic.
Merge branch '500-github-issue' of https://github.com/potomak/middleware into potomak-500-github-issue
send correct json body to github
Merge branch 'potomak-500-github-issue'