Support pre-processing directives and add env_brave

[Yuki2718]

I don't know if pre-processing directives are supported or not, brave-experiments/ad-block#114 is archived without any notion. In particular, if-condition to speficy Brave Shield (env_brave or something) will be useful to list maintainer. Sure, Brave has its own unbreak list, but if a Brave-specific issue like brave/adblock-lists#614 can be fixed in upstream lists by specifying Brave, this will save time of both Brave list maintainer (@ryanbr ) and me (or whoever filter author who mind Brave). Moreover, it will encourage more people to create and maintain lists for Brave. I believe if you can implement this, Adguard (@ameshkov) and uBlock Origin (@gorhill ) can add the same directive whose value will be false on them (this is completely different from uBlockOrigin/uBlock-issues#1213).