Add a few more tmux options to syntax highlighting

[rmunn]

The syntax/tmux.vim file is missing a few mouse-related options: mouse-resize-pane, mouse-select-window, and mouse-utf8. Since it's a one-line change, I've created a patch rather than make a Github pull request:

diff --git a/syntax/tmux.vim b/syntax/tmux.vim
index ae24be7..97c0b05 100644
--- a/syntax/tmux.vim
+++ b/syntax/tmux.vim
@@ -58,6 +58,7 @@ syn keyword tmuxOptsSet status-right-fg update-environment base-index
 syn keyword tmuxOptsSet display-panes-colour display-panes-time default-shell
 syn keyword tmuxOptsSet set-titles-string lock-command lock-server
 syn keyword tmuxOptsSet mouse-select-pane message-limit quiet escape-time
+syn keyword tmuxOptsSet mouse-resize-pane mouse-select-window mouse-utf8
 syn keyword tmuxOptsSet pane-active-border-bg pane-active-border-fg
 syn keyword tmuxOptsSet pane-border-bg pane-border-fg message-[command-]fg
 syn keyword tmuxOptsSet display-panes-active-colour alternate-screen