Revert Jison to 0.4.13

Jison releases later than 0.4.13 cause exceptions to be thrown when parsing
any code. Reverted to 0.4.13 as an easy fix.
Implement simple variable interpolation for string literals.

e.g. in the string "value is :{v}", the string ":{v}" is replaced with the
symbol v's value at runtime. Complex interpolation (i.e. full expressions)
are not yet supported. Interpolation only happens in strings defined as
literals, i.e. it won't occur for concatenating "value is :{" and  "v}".

Closes #1.