Allow wiggle room in SI prefixes that won't be ambiguous

Resolves: https://github.com/kitspace/electro-grammar/issues/18

Untested :)

Cool, could you add a test or two and re-build the demo site please.

Will do as soon as I'm on desktop. Seems this has been done in the antlr branch. Not sure I yet understand if/how these two ports will relate and coexist together. Will have to dig into docs later :)

Thanks, Antlr branch is work in progress but has been for a while now. It's fine to improve the current master branch. Test additions are especially important as when we do switch to Antlr we will re-use the test suite to make sure we aren't introducing regressions.