Allow the context selector struct name to be specified

[michiel-de-muynck]

You could already specify that no context selector should be generated, using context(false). Now you can specify the name of the context selector, using context(MySelectorTypeName).

Helps with #188. I mean, it doesn't do exactly what asked there, which was to be able to put the generated context selectors into a submodule, but in the comments of that issue it was suggested) that this could be an alternative solution for the same issues.

This is also the first step toward #199, where custom context selectors will be needed.

[shepmaster]

This is also the first step toward #199, where custom context selectors will be needed.

Can you explain a bit more about the general path you see? I don't follow how this plays into that bigger feature.