"Normalize" tags to the plural form

[Tiendil]

For example, text-editor -> text-editors

It will help to reduce the number of significant tags and standardize them.

The normalization means not rewriting existing tags or new tags but adding new plural tags if non-plural ones exist.

I.e., we need a new tags processor which will do this.

Before starting work on this task, a mechanism for tracking dependencies between tags should be implemented.