Exec once

More robust matching to provide some functionality from shifty that I missed. Now the exec_once command(s) will always fire, provided they're not already running.

My use cases: 
1. I have mocp automatically open when I hit my music tab, but i close the console when I'm done and let moc continue to play. When I go back to the tab, the console pops up again.

2. Always open three terminal emulators on my dev console, unless any are currently open.
```lua
exec_once = {
    "urxvt -name dev",
    "urxvt -name dev",
    "urxvt -name dev"
}
```