Add mediawiki-skin package type, modify mediawiki-extension package type

Name inflection for mediawiki-extension stays the same, except that a trailing
'-extension' is cut off if present.

Name inflection for mediawiki-skin will just cut off a trailing '-skin' if present.

That way package name collisions, e.g. between extension Bootstrap and skin
Bootstrap (they actually do exist both!), can be avoided.

The installation directory for skins is 'skins'.
Extract inflection of extension and skin package names into dedicated methods
Remove method descriptions

Method names are expressive enough on their own.
Merge pull request #141 from s7eph4n/master

Add mediawiki-skin package type, modify mediawiki-extension package type