Force utf-8 encoding when opening any file.

This should solve issue #1.

This is not the default encoding for io.open() as I thought.
Some systems might default to ascii or some other encoding.

Later I should add an encoding CLI argument to override the default
encoding.
Reset the window buffers properly after include

This should address issue #2.

The buf3 and buf5 window buffers were not properly reset after
an embedded include specification was consumed. Now resetting
them correctly.