Tweak focus() guards

* switching to inexistent buffers raises an exception and is thus
caught by the try block: remove the guard clause.
* the user can modify the line originally called: check it still
conforms to our starting status.
Make sure word to replace is unmodified

Also highlight it in the buffer for emphasis (if the user edits the line
so as to make a replacement impossible, the match highlight disappears,
thus providing visual feedback).
Added bare bones installation instructions