[feature request] ability to make your own modifier keywords

I noticed in [here](https://github.com/koekeishiya/skhd/issues/1) that you have added the modifiers `hyper` and `meh`.

It would be cool if we were able to create our own modifier keywords.

For example:

`mod1 = alt + shift`

So then, for example, instead of having to write:

```
# swap managed window
shift + alt - h : yabai -m window --swap west
shift + alt - j : yabai -m window --swap south
shift + alt - k : yabai -m window --swap north
shift + alt - l : yabai -m window --swap east
```

you could write

```
# swap managed window
mod1 - h : yabai -m window --swap west
mod1 - j : yabai -m window --swap south
mod1 - k : yabai -m window --swap north
mod1 - l : yabai -m window --swap east
```

The issue names no files or tests. Start by tracing how the existing hyper and meh modifiers are recognized and how configuration lines are parsed; done means supporting an assignment such as mod1 = alt + shift and using mod1 in the shown bindings.

Written by the indexing model from the issue text.