Custom grammars

[TheNightmanCodeth]

This PR adds an argument to .installPlugin that takes a set of custom Grammars to use while highlighting code in markdown.

Let's say we've created a custom Grammar called YamlGrammar. We can use it with this PR like so:

.installPlugin(.splash(withClassPrefix: "", withGrammars: [(YamlGrammar(), "yaml"), (SwiftGrammar(), "swift")])

This will apply the grammar to any code blocks in markdown prepended with "yaml", like so:

```yaml
...

I wrote a unit test but I did struggle getting Xcode to recognize the string to compare with. I had to escape some quotes and add newlines to get it working even though I noticed this wasn't done in the first unit test. Hope that's okay!

I also took the liberty of adding brief instructions to the README. Hopefully it reads well 😂