Add UseUsername to Mattermost bridge config

[icelander]

Is your feature request related to a problem? Please describe.

When I bridge from IRC to Mattermost, it always uses the Mattermost nickname and not the Mattermost username. This is a problem because Mattermost users can impersonate IRC users by changing their nickname. For example, Alice can set her nickname to bob in Mattermost, and in IRC it appears that this message is from bob.

Describe the solution you'd like

Add the UseUsername configuration option to Mattermost configuration exists Discord and Matrix.

Describe alternatives you've considered

I tried writing a Tengo script to query the Mattermost API but realized that was way too much work for something that should be built in. And also it didn't work.