Added new plugin: echo

I created a plugin to allow blind typing. It tracks non-duplicate (sequentially) key events, and passes them to a callback when a certain trigger/breaker character (only one, but it can be modified) is keyed. It passes it as both plaintext (respecting space, enter, backspace, tab) and an array of objects enumerated with character and modifiers.

It looked like there were some requests for something like this in the issues page, and I wanted something like this myself, so I made it as a plugin. Hope you like it!