Automatically add git coauthors when pairing on a call.

Updates the file $HOME/.gitmessage with each participant as a coauthor.

Ensure that $HOME/.gitmessage shows up in your commit template with the following command:

$ git config --global commit.template $HOME/.gitmessage

For Windows you can use:

> git config --global commit.template %USERPROFILE%\.gitmessage