Support for alerting by shell commands

Hello.

Firstly, want to say thank you for great software.

When I found this application, I liked almost everything in it, except only that I couldn't make custom alert methods.

Exec for every rule is hard way to make custom alert, so I decided to extend alert functions. This commit adds new tag in configuration language: alertcommand, that executes shell command indicated in alertcommand tag. For example:

executes notify script with description of event.

We can specify event types and set reminder by analogy with mail alert.

I implemented it on my server, now it works like I want. So, I decided to make pull request. Of cause it's your decision to add it or not.