New drone pushover plugin

Hi bradrydzewski, in your http://readme.drone.io/0.5/custom-plugins-in-bash/ example, there is a bug. I’m not a bash programmer so it took me a while to figure out the unsuspected error. It is fast to create a drone plugin though. Congra for the great piece of work.

-[ -n PLUGIN_USER ] && PLUGIN_USER="${WEBHOOK_USER}"
+[ -n PLUGIN_USER ] && PLUGIN_USER="{WEBHOOK_USER}"

PS: I know why, you don’t have a bug, but it is missing single “$” in front of the PLUGIN_USER due to how ?markdown is rendered.

it looks like there is already a drone-pushover plugin but it is not on http://plugins.drone.io/ webpage.

Feel free to submit the docs for either/both plugins to this repo for listing on plugins.drone.io.