Allow rewrite of categories like frontmatter_fields

[natrius]

For a large blog it might be usefull to be able to rewrite the categories like its done with the frontmatter_fields.

exports.frontmatter_fields = [
	'title',
	'author:authors',
];

This rewrites the author to authors and something similar could be done for the categories.

exports.filter_categories = [
       'uncategorized',
       'category1:categoryX',
       'category2:categoryX',
];

Also allow that some categories could be merged.