Removal of obsolete code, QOL features, Bugfixes

• Removed the #IFELSE keyword
• Changed the #ELSE keyword so that it has the be written as a child of an #IF keyword. See https://github.com/JPVenson/morestachio/wiki/Keywords#else
• Changed the behavior of the IfDocumentItem and added the Inverted property so it can replace the IfNotDocumentItem
• Added the #ELSEIF keyword as an conditional else within an #IF keyword. See https://github.com/JPVenson/morestachio/wiki/Keywords#else
• Removed the obsolete .() ToString formatter. This is possibly a breaking change if you did not update your formatter calls earlier. Just insert .ToString() to get the same behavior
• Reduced XML serialization size for partial documents
• Fixed: /DECLARE tags would not be correctly reconstructed using the ToParsableStringDocumentVisitor
• Added ParserOptions.UnmatchedTagBehavior
• Added $services variable for direct access to services instances from template
• Added Optional FileSystemService for direct file access from within the template