Implement `console` command section property

[bastimeyer]

As described in #169

Adds an optional console property to the command sections with default value true. If set to false, the command's launcher will be built from one of the GUI launchers of simple_launcher and a shebang with pythonw.exe will be included instead of python.exe.

Changes:

• extend configreader with new console command property
• copy both launchers as dat-files (launcher_exe.dat and launcher_noconsole_exe.dat)
• split between command-append.zip and command-append-noconsole.zip, depending on the command's console value
• rewrite _assemble_launchers.py to reflect these changes and change its argvs so that the shebang can be tested
• add/rewrite tests
• add documentation

I haven't actually built any installers with these changes yet, but the updated tests should assure that this is fine. There are also a couple of things which could be improved in the future, like importing file name constants for example.

[bastimeyer]

Just built a couple of installers and noticed a quotation mistake in NSIS' _assemble_launchers call. That's fixed now.

[bastimeyer]

@takluyver Is there a chance for a review this week? I would really love to get this issue out of the way as soon as possible. Thanks!

[takluyver]

Sorry for the delay; as I hinted, this isn't part of my day job, and I don't always feel like more work with code after spending the day doing that. Taking a look now.

[bastimeyer]

Any further changes, or is this good to go? :)

[takluyver]

Thanks! :smiley: