Virtualize the logger instantiation

Add a _logger function in Module::Logger, which creates the logger
instance, making it easier to subclass the Logger module.

Add a Logger::StdErr module, which dumps everything to STDERR and
supporst no min_level, i.e. it will print all levels. This module is
convenient to use during development with Kelp::Less.
Fix debug and error keywords in Kelp::Less

They did not work well if no logger module was installed.
Update bin/Kelp

- Less application will create a psgi file with the specified name
  (previously it would always create app.psgi)
- Existing files will not be overwritten, unless --force specified.