Add initial documentation framework for TypEr.
Prepare release
Correct the name of a compiler option and fix the corresponding warnings

The option `warn_export_vars` was erroneously written as `warn_exported_vars`
in Makefiles of dialyzer and typer. As a result, `erlc` was silent about such
warnings (why on earth doesn't it complain on unrecognized options??) which
were corrected as part of this change.
Merge branch 'kostis/dialyzer-warn_export_vars' into maint

* kostis/dialyzer-warn_export_vars:
  Correct the name of a compiler option and fix the corresponding warnings
Prepare release