Test release for SeaMonkey 2.53.13 (and as below)

• compatibility with "last" Firefox 57 (stylish-userstyles#319)
• update version number
• update compatibility

Adapts https://github.com/Lootyhoof/stylem/releases/tag/v2.2.7 to this repo:

• support Pale Moon 30
• minor internal cleanup
• restore SeaMonkey support.