Fixes #168: Don't include config.h unless WEBVTT_NO_CONFIG_H is false

This change provides a mechanism for Mozilla to build our library
without having to patch util.h, which is nice.

Building with CPPFLAGS += -DWEBVTT_NO_CONFIG_H=1 will allow them to
build without trying to include generated config headers.
Merge pull request #168 from caitp/issue-168

Add direct support for building without generated headers