Add STDERR argument, and write docs for args module

[zachriggle]

This should make it so that anybody can choose where the logs go without needing to set it every time.

#!/usr/bin/python2
from pwn import *

log.info('before')
pwnlib.log.console.stream = sys.stderr
log.info('after')

$ ./foo.py
[*] before
[*] after
$ ./foo.py > /dev/null
[*] after
$ ./foo.py STDERR > /dev/null
[*] before
[*] after
$ export PWNLIB_STDERR=1
$ ./foo.py >/dev/null
[*] before
[*] after
$ ./foo.py 2>/dev/null
# <nil>

Fixes #759

[zachriggle]

It appears that this is superfluous, since we instantiate console with sys.stdout as its stream just a few lines above.

[zachriggle]

This is not actually superfluous, it causes tests to fail. Wat.

[zachriggle]

Looks like this was accidentally keeping our unit-tests from breaking by re-invoking install_default_handler() for each module's doctests. The root-cause is pwnlib.commandline.common was setting it to stderr, which should only happen when main() is invoked, not when the module is imported.

[zachriggle]

The tests are breaking because the command-line programs are generating doctests, which change the log to stderr.

[zachriggle]

CI passes, I'm choosing to ignore the Codacy issue since I'm not sure how to resolve it (you can't have properties on classmethod).