Reuse the BlackFriday instance when possible

Reuse the BlackFriday instance when possible

This is in heavy use in rendering, so this makes a difference:

```bash
benchmark                                                                                    old ns/op     new ns/op     delta
BenchmarkSiteBuilding/TOML,num_langs=1,num_pages=500,tags_per_page=5,shortcodes,render-4     124551144     107743429     -13.49%

benchmark                                                                                    old allocs     new allocs     delta
BenchmarkSiteBuilding/TOML,num_langs=1,num_pages=500,tags_per_page=5,shortcodes,render-4     528684         435118         -17.70%

benchmark                                                                                    old bytes     new bytes     delta
BenchmarkSiteBuilding/TOML,num_langs=1,num_pages=500,tags_per_page=5,shortcodes,render-4     53306848      45147832      -15.31%
```