Expose the ES6 module transpiler compiler options

This allows users to take advantage of current and future options in the
Compiler constructor. The one I was most interested in (and led me to write this
commit) is `compatFix` which allows transpiled files to work seemlessly with
non-transpiled. For example:

```rb
ES6ModuleTranspiler.compiler_options[:compatFix] = true
```
Add note about `compiler_options` to README and CHANGELOG
Merge pull request #8 from caseywebdev/compiler-options

Expose the ES6 module transpiler compiler options