support external jsmn implementations

[dangmoody]

Hi,

This is my first time making a PR/contribution to someone else's project - so please forgive me for any schoolboy errors.

I'm using CGLTF in a personal project where I am also using JSMN on its own to parse some JSON files. My project builds via Unity Builds, so I was getting an error that JSMN code was already being defined in another source file - it was the embedded JSMN implementation inside cgltf.h.

I put in a change for myself where passing a define in before including cgltf.h doesn't use the embedded JSMN implementation but then thought these changes might be useful in the library for others?

I added a quick test to make sure that it still compiles with the define turned on (CGLTF_EXTERNAL_JSMN) and with the external jsmn file included. Everything passes on Travis.

One thing I did notice was that the JSMN implementation seems to be a bit behind the latest version, which required a typecast at cgltf.h line 5072. Other than the #define I've added and the bump in version number (I don't know what your convention for version numbers is with this project but I can always revert it if you like), that's the only other change I've made.

The only thing I've not done is update the documentation. I figured it was only worth me doing that if you OK'd the change. =)

I'd be very interested to hear your feedback on this. I appreciate this is probably an edge-case problem - but hopefully this will be useful?

Thanks,