Add modifiers config option

[innovate-invent]

Resolves #42

I am not actually sure what the use case is here for this. @drjaska would you mind testing this?

[wez]

I'm also not sure what problem is being solved here, so I'm inclined to sit on this one for now!

[drjaska]

@innovate-invent I would reword the newly added text in README.md to

If you want to override the default modifiers (listed below)
you can set a custom set of keys as the modifier keys via the modifiers = [] config option.

from

If you want to override the default modifiers (KEY_LEFTALT, KEY_LEFTCTRL, etc.)
you can list all the modifier keys via the modifiers = [] config option.

but otherwise LGTM. This patch allows users to set custom modifier keys without source code edits which allows for configs suchs as the following to not litter random unremapped keys anymore. Before this patch pressing Capslock, W and S at the same time would let the S presses through but this patch allows the keypresses to be remapped properly.

modifiers = [
  "KEY_CAPSLOCK",
  "KEY_FN",
  "KEY_LEFTALT",
  "KEY_RIGHTALT",
  "KEY_LEFTMETA",
  "KEY_RIGHTMETA",
  "KEY_LEFTCTRL",
  "KEY_RIGHTCTRL",
  "KEY_LEFTSHIFT",
  "KEY_RIGHTSHIFT",
]

[[remap]]
input = ["KEY_CAPSLOCK", "KEY_W"]
output = ["KEY_UP"]

[[remap]]
input = ["KEY_CAPSLOCK", "KEY_A"]
output = ["KEY_LEFT"]

[[remap]]
input = ["KEY_CAPSLOCK", "KEY_S"]
output = ["KEY_DOWN"]

[[remap]]
input = ["KEY_CAPSLOCK", "KEY_D"]
output = ["KEY_RIGHT"]

[[remap]]
input = ["KEY_CAPSLOCK"]
output = ["KEY_F24"]