Add mod support (eg ISMC) as an option for Sandstorm Server

## User Story

As an upcoming Insurgency Sandstorm server owner, I want to have an option for lgsm to automatically deploy ISMC as a mod onto the gameserver so that installation will be simplified and especially beginner Ins:Sand server owners can set it up more easily. Also ISMC is becoming the driving force for Sandstorm which would solidify Linux with lgsm as a solid decision.

## Basic info

* **Distro:** [Ubuntu 20.04][Debian 9][CentOS 8]
* **Game:** [Insurgency: Sandstorm]
* **Command:** Maybe `installflag`?
* **LinuxGSM version:** not implemented yet

## Further Information

https://insurgencysandstorm.mod.io/ismcmod1
Mods are loaded through mod.io. It would be advisable to have further options for additional mods like Improved AI, server info, and maps:
https://insurgencysandstorm.mod.io/improvedai
https://insurgencysandstorm.mod.io/motdreworked
e.g. https://insurgencysandstorm.mod.io/sultan-of-drugs

Could imagine a config file which gets filled with either workshop or mod.io ids/links for downloading stuff and put onto the server as a complete new feature on supported gameservers.

## Further Reading

Anything needed to know for mod installation on Sandstorm is added here: https://insurgencysandstorm.mod.io/guides/ismcmod-installation-guide

The mods/startmap needs to be loaded as startup argument. But this could be done by a config variable which needs to be set. This also could be a vanilla map as an easy default. Admins could change that to mod maps they've added.

A great documentation about how to add maps and include them is given by this map description: https://insurgencysandstorm.mod.io/sultan-of-drugs

Thanks for the work guys, let me know if I can be of any help.