Add support for modifier keys in isPressed

[cmawhorter]

Adds support for string modifier keys to isPressed.

Currently, key.isPressed('command') does not work when key.isPressed(91) does.

[vincens2005]

I've merged this here
As I use this library and it hasn't been touched in two years, and has many prs and issues, I'll be maintaining it in my fork.

[cmawhorter]

@vincens2005 you might be interested in this too which adds sequences/chords to keymaster. i used it in production for a while but don't use it any longer (mainly because keymaster became ded)