Allow for including options from parent `argparse.ArgumentParser`
objects in preparation of splitting out the handling of preliminary
options from the `OptionManager`.
When calling `ArgumentParser.parse_args()` with the `namespace`
argument, command-line options are just added to the namespace without
going through any of the argument parsing and type conversion logic
(e.g., the `type` keyword argument of `ArgumentParser.add_argument()`).
In other words, it is assumed that a namespace is well-formed from a
previous invocation of `ArgumentParser.parse_args()`.
The `values` parameter is intended to be values already-provided from
configuration files. To take advantage of the logic defined by
`ArgumentParser.add_argument()`, utilize
`ArgumentParser.set_defaults()` instead.
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
On Flake8 2.x we added the information about the implementation,
version, and operating system to the --version output to make helping
users easier. In short they can pretty simply just give us the output
from
flake8 --version
And we can get a lot of the information that we need.
This relies on two things:
1. Properly configuring flake8-import-order to use that style
2. Properly configuring flake8-import-order to know that flake8 is our
application name.
