Improve custom plugin support

[reck1610]

What is the issue, e.g. map voting, you would like to solve?

In my opinion, custom plugins have 2 problems at the moment.

1. Custom base classes for plugins will always be loaded as a plugin.

This might not cause a problem, but it causes custom plugins to be handled differently from default plugins.
Currently, the default base classes are blacklisted in the plugin loader.

2. Custom plugins have to be placed in a hard-coded location in the repo

This causes git changes and makes updating SquadJS harder.
This also reduces maintainability for both custom and default plugins, if custom plugins are used.

How would you like us to solve this issue?

Problem1: I would like to introduce the file extension .squad.js for plugins. This removes the need for blacklisting other files in the plugin directory.

Problem 2: I would like to make it configurable from where SquadJS is loading the plugins. Furthermore, I would like to allow multiple paths to be configured in order to increase maintainability for both custom and default plugins, if custom plugins are used.
I would like to introduce pluginPaths into the server configuration like this:

{
  "server": {
    "id": 1,
    "host": "xxx.xxx.xxx.xxx",
    "queryPort": 27165,
    "rconPort": 21114,
    "rconPassword": "password",
    "logReaderMode": "tail",
    "logDir": "C:/path/to/squad/log/folder",
    "ftp": {
      "port": 21,
      "user": "FTP Username",
      "password": "FTP Password",
      "useListForSize": false
    },
    "pluginPaths": [
      "squad-server/plugins/",
      "/opt/custom-squadjs-plugins/"
    ]
  }

Both relative and absolute paths should be allowed. Plugins will be loaded in order (starting with index 0), if a plugin already exists it will be replaced. This should be logged accordingly.

Additional Info

I am already working on this.
This relates to #275.

[werewolfboy13]

Please see attached PR. Closing as we are not able to proceed with this major change but will keep in mind.