Highlighting trailing whitespaces

Added option `hltrailingws` for highlighting trailing whitespaces
at the end of lines. Note that it behaves in a "smart" way.
It doesn't highlight newly added (transient) trailing whitespaces
that naturally occur while typing text. It would be annoying to
see transient highlighting every time we enter a space at the end
of a line while typing.
So a newly added trailing whitespace starts being highlighting
only after the cursor moves to another line. Thus the highlighting
serves its purpose: it draws our attention to annoying sloppy
forgotten trailing whitespaces.

runtime/help/options.md

@@ -181,6 +181,12 @@ Here are the available options:
 
 	default value: `false`
 
+* `hltrailingws`: highlight trailing whitespaces at ends of lines. Note that
+   it doesn't highlight newly added trailing whitespaces that naturally occur
+   while typing text. It highlights only nasty forgotten trailing whitespaces.
+
+	default value: `false`
+
 * `incsearch`: enable incremental search in "Find" prompt (matching as you type).
 
 	default value: `true`