Interpolate fix

This fixes #92. It's a bit of a hack I think, but I believe that it should only impact people using a different start or end symbol than the default which is hopefully a relatively small number.

I noticed that using `[[ ]]` broke one of the *custom* templates in our demo, but using a different start and end symbol made everything work fine. I expect that people who do need this functionality will just have to work around using different symbols and the funny things it does. I believe this PR should be merged, but I want @astrism to look at it first.