Use `copy-mode -q` to leave copy mode before running a command

[mvanderkamp]

Fixes #230

Four years ago tmux introduced a -q flag to copy-mode: https://github.com/tmux/tmux/commit/06c3079d66929e0c71575e877098fc533ae5f4a5

Using copy-mode -q on the runner instead of relying on the q mapping, which could be remapped, should be more reliable.

This updates the default value of VimuxResetSequence from "q C-u" to simply "C-u".

This will pose problems for users on a tmux version older than the introduction of the -q flag (before version 3.2a). We could perhaps catch the unknown flag error and try using q in those cases.