0.5.0 Feature Release

Release 0.5.0 aims to reach feature parity with C++ FlatBuffers as of
end 2017. These new features are union vectors and mixed type unions
that can include tables, structs and strings, and type aliases for int8,
uint8, int16, uint16, int32, uint32, int64, uint64, float32, float64
types in the schema. Support for base64(url) JSON encoded [ubyte]
vectors has been added which will also be added to Googles flatc tool in
a future release. In addition the following changes have been added:
Runtime builder library support for aligned_alloc/free. Handling of
unions is slightly incompatible with previous releases as covered in the
documentation and the changelog. v0.5.0 fixes a bug that could cause a
JSON parser to reject some valid symbols for some schemas and also fixes
a non-critical JSON scoping issue with symbolic union names and a bug
verifying buffers with a struct as root. Low-level custom frame support
has been improved in the builder library which is useful for complex
parsing scenarios.