[WIP] Fix and refactor launcher (part 1)

### Motivation
Improve the consistency and reduce the verbosity of the launcher; use `argparse` more aggresively. There will be about 3 or 4 PRs, each reducing the launcher size by about 100 lines.

### Changelog
* Fix various bugs and typos.
* Structure the parser arguments in a way that is easy to read and extend.
* Name the parser arguments, the `defaults` keys and the `OpenBazaarContext` atttributes with consistent names. Some flags are removed, others are renamed; consult the `usage` message.
* Reuse the `Namespace` object returned by the parser; it has all the default options built in.
* Remove execute flag from various `*.py` files.
* Remove outdated Windows script.
No need to convert. I will restore the original design that had `seeds` be a `list`. As for duplicates, the duplicates are due to a semantics issue: If I specify `--seeds X Y`, does that mean I *only* want `X` and `Y` as seeds or do I want to have `X` and `Y` *in addition* to all built-in seeds? I 'd prefer the first option, thought the latter may be more user-friendly.