Add custom Discord notification

[dirtboll]

This adds the ability to provide Discord notification template using JSON via discord_message_template environment variable. Example inside a bash script:

set -a
discord_webhook=https://discord.com/api/webhooks/blablabla
# Generated from https://discohook.org/
read -r -d '' discord_message_template <<- 'EOF'
  {
    "content": null,
    "embeds": [
      {
        "title": "<a:furnace_angry:839235629454983218> Detected Malware `${malware-family}`",
        "color": 16711680,
        "fields": [
          {
            "name": "Server",
            "value": "My Infected Server",
            "inline": true
          },
          {
            "name": "File",
            "value": "`${file-path}`",
            "inline": true
          },
          {
            "name": "Malware type",
            "value": "[${malware-platform}.${malware-type}.${malware-family}.${malware-variant}](https://www.google.com/search?q=${malware-platform}+${malware-type}+${malware-family}+${malware-variant})"
          },
          {
            "name": "Class path",
            "value": "`${malware-class-path}`",
            "inline": true
          },
          {
            "name": "[Line] SourceFile",
            "value": "**[**${malware-line-number}**]** `${malware-source-path}`"
          }
        ]
      }
    ],
    "username": "Minecraft Malware Scanner",
    "avatar_url": "https://i.imgur.com/i9GSb45.png",
    "attachments": []
  }
EOF
java -jar MCAntiMalware.jar --notify discord --singleScan true

Available format parameters:

Parameter	Replaced with
${file-path}	Path to the infected file
${malware-platform}	Malware platform
${malware-type}	Malware type
${malware-family}	Malware family
${malware-variant}	Malware variant
${malware-class-path}	Class path containing the malware
${malware-source-path}	Path to the source code containing the malware
${malware-line-number}	Line number of the source code

If not using template, you can still customize username and avatar URL using discord_username and discord_avatar_url environment variable.

[OpticFusion1]

There is no reason to have to re-compile this on every method call, convert the Pattern to a CONSTANT.

[dirtboll]

Should be resolved.

[dirtboll]

Sorry, I refactored it a bit. What do you think?