Change default module to Main

[rikhuijzer]

This PR fixes an inconsistency in the behaviour of @sco and sco. The former used to evaluate code inside M::Module by default, whereas sco evalutes code in Main::Module by default. Given that normal usage on the REPL evaluates everything in Main, I decided to make that the default.

This means that code which used to write f() where f is a function inside MyModule should be replaced by MyModule.f() or M.f() after M = MyModule is set.

It would also be possible for users to export f inside MyModule to allow calling f() directly, so Main is definitely the better choice. Note that dependencies from MyModule can easily be made available to Main by using @reexport.

Also, due to the more capable gen(::Vector), gen(::Function) is now removed.

Also, with this PR, users can also more strictly enforce no auto-detection for captions or labels by setting nothing. For missing, auto-detection will occur.

Closes #130.