Add ability to toggle/hold key

[khyperia]

Fixes #926

Adds a new togglekey() action, which holds a key until the binding is pressed again.

[khyperia]

My use case is that in the game Final Fantasy 14, you need to hold right click to pan the camera around. This is annoying, so on Windows, I use the program X-Mouse Button Control to bind scroll-left to toggle holding right-click. With this patch, in keyd this now looks like:

[main]
scrollleft = togglekey(rightmouse)

Please feel free to reject this PR if it doesn't fit! I did this purely for my own personal use case, and am only submitting a PR in case it's useful for other people and you think it's valuable for keyd. I'll just continue running my fork if not! (And feel free to suggest any feedback or mistakes to help this PR get in.)

Thank you for your work on keyd! It's a super awesome tool~