Clarify docs for custom logger backends

The docs currently make it seem (to me, at least) that
`wrap-with-logger` accepts a single map argument to define custom
loggers, when in fact, it accepts a variable number pairs of level
keywords and logger functions.

Here's an attempt to update the Readme to make this more clear, if you
like it :)
Remove accidental markdown; clojure-ify
Fix typo, clarify logging functions
Make examples consistent
Merge pull request #12 from fajpunk/clarify-custom-logger-backend

Clarify docs for custom logger backends