Support arbitrary script execution during countdown about termdown

[EvansJahja]

Here's my attempt to this
#45

Example usage:
python3 termdown.py --exec-shell "espeak -v en-us" 10

the current second is put as the last positional argument, so the above would result to:

espeak -v en-us 10
espeak -v en-us 9
espeak -v en-us 8

[pigmonkey]

Yup, that's pretty much how I imagined it!

I think the only advantage to using environment variables is that, in the future, more context (like the critical seconds, or the value of some new flag that termdown was called with) can be passed to existing external scripts without breaking them. If those scripts depend on positional arguments, and termdown later changes the number or position of arguments it passes, it could break things for users.

[pigmonkey]

I've been using #45 for a bit now. It satisfies my use case, and I think greatly extends the capability of termdown. Is there any chance of getting something like this merged?