Allow turning off string expansion

[tjvr]

It doesn't make sense to expand string literals into characters when using a custom tokeniser.

For the built-in tokeniser, literal strings should instead match the value of the token. (The %-syntax must be used for matching the token's type.) This change is useful for e.g. tosh, where I use literal words all over the place, or more generally for matching specific keywords.

Relates to our solution for #166. Only really makes sense after #198.