Allow unquoted strings consisting entirely of number chars

Previously, a string which was an invalid number (such as
"1.0.0" or "1e3e3") but consisted entirely of chars allowed
in numbers would be an error and require quoting. Now, we
allow such strings to be unquoted.
Merge pull request #116 from typesafehub/wip/havocp-allow-unquoted-number-chars

Allow unquoted strings consisting entirely of number chars