Update LevelFormatter docs and add tests for formatting string.

[jkulhanek]

In this PR the fix for #84 is implemented with tests.

[borntyping]

This looks great, though has broken tests on Python 2:

py27-test run-test: commands[0] | py.test -v colorlog
ImportError while loading conftest '/.../borntyping/python-colorlog/colorlog/tests/conftest.py'.
  File "/.../borntyping/python-colorlog/colorlog/tests/conftest.py", line 74
    def function(*args, lines=None, **kwargs):
                            ^
SyntaxError: invalid syntax
ERROR: InvocationError for command /.../borntyping/python-colorlog/.tox/py27-test/bin/py.test -v colorlog (exited with code 4)

(Sorry, at some point automated tests for this repository have broken. Tests can be run with Tox, though need various versions of Python installed.)

[borntyping]

Tests that this works as expected have been added in 6.0.0a2.

(This is a prerelease and may change before version 6.0.0.)