Some functionality working

A partial tokenizer, but it seems I'll have to make somewhat of a tokenization tree (AST) in order to do all the features I want.
Tests
Support multiline strings
Put brackets around all substitutions

I'd like to only put them round stuff that needs it, but this is easier for now.
Amend tests for more coverage
`#{}x` and `#{}n`
Inline HTML tests

Not sure why I hadn't written these already
Fix HTML substitution