New feature: possibility to add custom messaging systems

[123unix]

We can see a lot of feature requests for various messaging systems. All those systems seem to differ with the three basic attributes:

1. Name
2. icon
3. URL pattern

And here's the idea: let's create a facility to add arbitrary messaging systems.

This way not only any new system will be addable without the need for a new release, but also users will get a feature to configure only systems they want to use, not everything imaginable cluttering the screen.