Allow mute commands to update duration of the mute

**What component of Red (cog, command, API) would you like to see improvements on?**

Mutes

**Describe the enhancement you're suggesting.**

Currently, there's no clear way of changing the duration of an existing mute that doesn't involve unmuting and muting with a new duration. It does seem that it already works with `[p]mute` when using role-based mutes, but it seems to be work incidentally.
I suggest that all mute commands get an enhancement that would allow the commands to update the duration of an existing mute rather than fail with "That user is already muted in this channel." or a similar error message.