Correct messageGetter option to MessageProvider

The `messageGetter` was called with a params array as second parameter.
That behavior matched the documentation, but the function usually passed as
`messageGetter` (`mw.msg`) expects the params to be given as individual
arguments.
Factor inputextender and content out of time and globecoordinate experts

This introduces ExpertExtender with some modules. It makes it possible to reuse
the inputextender in other experts, like quantity or monolingualtext.
Rename initialShow -> onInitialShow
Add small entry to README
Merge pull request #51 from wmde/refactorExperts

Factor inputextender and content out of time and globecoordinate experts