Features about renaming and merging batch of tags

[lucazav]

It'd be really epic if this extension will provide the following features:

• Rename a tag for all URLs or only selected ones by existing tags
• Merge one or multiple tags in a new one (e.g. the tags "advanced" and "analytics" will be merged into the only one tag "advancedanalytics") for all URLs or only selected ones by existing tags
• Delete a specific tag for all URLs or only selected ones by existing tags
• Add a new tag only for all URLs or only selected ones by existing tags