Fix texcoord export.
added clearing of unknown parameters map

not doing so caused undefined behavior in objects with multiple
materials using custom parameters.
Merge branch 'master' of https://github.com/caryanne/tinyobjloader
Cppcheck suggested performance improvement

variables should be passed by reference.
They are passed by value. It could be passed as a (const) reference which is usually faster and recommended in C++.

This gives a 9% performance boost in my project.
26.28s to load instead of 28.94s
Update readme.