string parser does not recognize uncompliant strings

[nlohmann]

The Readme currently states:

Furthermore, we allow for more escape symbols in strings as the JSON specification. While this may not be a problem in reality, we are aware of it, but as long as we have a hand-written parser, we won't invest too much to be fully compliant.

And in fact, anything between two unescaped quotes will be treated as strings. However, the JSON standard is very explicit how a string may look like:

As long as the class does not implement this string specification, the class is not fully JSON compliant.

[nlohmann]

The parser of https://github.com/dropbox/json11 could give some ideas how to write a compliant parser manually.

[nlohmann]

Also http://re2c.org may help (as pointed out by drac667).

[nlohmann]

Fixed in latest commit: re2c is used.