Add `.clearpronouns` command

### The problem
Users might set their pronouns by mistake or just to test the functionality and then they are stuck.

### The solution

Add an "unsetpronouns" that deletes pronoun information for the nick. 
Something like this might work. 

```python
@plugin.command('unsetpronouns')
def unset_pronouns(bot, trigger):
        bot.db.delete_nick_value(trigger.nick, 'pronouns')
```
