Revamped module loading, addresses #202 issue

[ghost]

Revamped the module loading to be shorter and avoid certain bugs noted in #202

Old code features:

• Always includes default module folder
• Files inside default folder are subject to some validation (Don't start with "_" and end with ".py")
• No validation if whitelisted modules exist in default folder
• Direct file paths inside 'extra' are subject to no validation
• Files insides folders, which are inside 'extra', are subject to no whitelisting/blacklisting logic
• Files insides folders, which are inside 'extra', are subject to some validation (Don't start with "_" and end with ".py")
• Only files inside default folder are subject to blacklisting/whitelisting

New code features:

• Always includes default module folder
• All files insides folders need to exist
• All files insides folders are subject to some validation (Don't start with "_" and and end with ".py")
• All files inside folders are subject to blacklisting/whitelisting
• Direct file paths inside 'extra' are subject to no validation

[ghost]

Slightly relevant:
How does "overloading" modules sound? Basically instead of keeping an array of filenames:

filenames = [ "default/admin.py", "default/ask.py", "extra/admin.py" ]

We keep a dictionary<name, filename>, so that files that have the same name override each other instead of loading twice:

filenames = { "admin" => "extra/admin.py", "ask" => "default/ask.py" }

In the above example, since extra/admin.py was validated last it replaced default/admin.py.

[ghost]

I may have forgotten to rebase before pushing, hope that's not an issue, I was only 2 commits behind (the typo fix).

[kaneda]

You could make this clearer

[ghost]

I feel it's a lot more clearer than it was before.

The excluded and enabled variables aren't called "excluded_modules" and "enabled_modules" because it makes the lines really long, but if that's not a problem I can gladly rename them.

fn can probably be renamed to file, or filename.

Besides that I sadly can't think of a way to improve the code.

[kaneda]

@Bekey one comment, do rebase against latest master and force push