* Use set literals instead of set([...])
* Avoid list(sorted(...)) as sorted returns a list
* Replace dict() with dict literal

By ordering the plugins, --version becomes reproducible so that it
continuously prints the same output (provided the plugins have not
changed).

Closes #297

When invoked it will print out JSON that has all of the debugging
information needed by the maintainers to diagnose or reproduce a bug.

Closes #207

Previously, all testing was done from the directory in which the
configuration file lived, so this bug went unnoticed. However, if you
run Flake8 against its own source from a directory above, you would
notice that the patterns in the exclude config value in tox.ini were
ignored. This is because we (like any reasonable person) are using
relative paths. The path is relative, however, to the directory in
which the configuration file was located. So we keep track of which
directory that is and use that to normalize the paths in the config
file.

Yes, there is an unrelated change to our tox.ini in this commit as
well. ;-)

Closes #194

Previously, to ensure that plugins on by default were reported, we
added them to the select list. This means that ignoring them became
impossible. To accomodate our reporting logic and a user's ability
to ignore, we need to keep our select and extended select lists
separated.

This allows us to have a better understanding of who is selecting what,
where, and how and make our decision as to whether or not an error
should be reported more wisely.

Closes #195

Flake8 3.0 was stopping once it found the current directory but the
historical behaviour (that we didn't intend to break) searched past
that (towards root) until it found one of the project/local config
file names that could be read.

Closes #181

If a user specified `--max-complexity` on the command-line, they
would be told that it did not exist. The same would be true of any
option provided by a plugin. This is because we parse the command-line
arguments twice in Flake8 -- the first time to specify the verbosity
and destination for logging, the second time to actually execute Flake8.
Since plugin options are not registered to start with the first time,
they are not valid options. So when we first parse the options, we should
only attempt to parse the ones which we know about.

Closes #168

When flake8's config is in setup.cfg, setuptools attempts to set those
options on the command instance. If they don't exist, it fails early
complaining that a specific option does not exist.

This adds this back and does it better than the Flake8 2.x version.

Closes #163