Fix Center Window Frame function

[jaidetree]

Fixes #119

Allows you to add something like the following to your config.fnl:

{
  ;; ...
        :modules {:windows  {:center-ratio "50:60"}}
  ;; ...
}

Then you can use the center-window-frame window modal action to use it like the following:

Creates an advisable function called windows.position-window-center that takes the center-ratio config string and the target window. It is responsible for parsing the center-ratio string into numbers, and applying them to the target window. The advisable function gives users the option of overriding it to parse the center-ratio string in a different format and apply the calculation differently if needed.

If an option is not set, defaults to 80:50 which is a percentage of the screen. This is nearly identical to the previous version defaults.

[Grazfather]

I think the screen should probably be passed in.

[Grazfather]

Maybe make this position-window-center-on-screen, though I think the name now is fine.

[jaidetree]

I thought about doing that, but was thinking that really depends on the implementation. The default behavior is to use the active screen, but users may want it to cycle between screens or pick the main screen, or any other logic to select a screen. The ratio-str and window are the only facts that are safe to assume for anticipated use-cases.

Though I suppose it could accept the current screen as a default and not use it but if you are aiming to implement advice then it's kind of a waste to have it query the screens again twice.

[Grazfather]

By making the screen an arg we assuming less. We can make center-window-frame provide (hs.screen.primaryScreen) by default.

By having more of the knobs in the args it also makes advising easier, since you can write one that just modifies the arguments instead of having to duplicate the logic.

[jaidetree]

You have a good point there. Pushed!