Add custom fmj field parsing

[supersaiyansubtlety]

Based on #23
Implements my suggestion here #20 (comment)
IMO if this is added the config added by #23 should be removed, as installing the mod would be 'enabling' it.
It only makes sense to merge this if the mod is hosted on Modrinth+CurseForge, so users can install to 'enable'.

The fmj field has several forms:

• use mod id for Crowdin and Minecraft project names

"custom": {
    "crowdin-translate": true
}

• use the string value for Crowdin and Minecraft project names

"custom": {
    "crowdin-translate": "some-mod"
}

• specify all the params for CrowdinTranslate#downloadTranslations(String crowdinProjectName, String minecraftProjectName, String sourceFileOverride, boolean verbose); sourceFileOverride, and verbose are optional

"custom": {
    "crowdin-translate": {
      "crowdinProjectName": "some-other-mod",
      "minecraftProjectName": "some_other_mod",
      "sourceFileOverride": "thing.json",
      "verbose": true
    }
}