Add include resolver and remove automatic inclusion of files with specific file extensions.

[TeresiaOlsson]

I have made the changes to switch the behaviour of the resolvers for including files discussed in #437.

This is what I did:

• Renamed the file resolver to include.
• Removed that strings with some specific suffices would automatically be interpreted as files to load and expand. Now you need to explicitly write ${include: ...} for it to be loaded and expanded.
• Updated the tests accordingly. That includes updating all the config files used for the tests since they make heavy use of this.
• Keep the path resolver for now since I know there are things in the documentation which uses it. It should be possible to remove it after the documentation tutorials are updated.

Only part I found maybe not so nice is that you now need to use ${include: ...} also for the catalog but I think that only affects the static catalog?