Added “Update Tag” action

When invoked, user is asked for an abbreviation to update existing  (under caret) tag.

In this abbreviation some characters has special meanings, e.g. users can precede attribute/class names with `+` or `-` to either append or remove value.

Example abbreviations:

`.+c1.-c2[-title]`: Add `c1` class name, remove, `c2` class name and `title` attribute
`span[+title=text]`: Rename tag to `span` and append `text` to `title attribute

Closes #244