Update Lemon so that it accepts filename arguments to directives like

Update Lemon so that it accepts filename arguments to directives like
"%include".  Ex:  "<tt>%include &lt;./subdir/file-to-include.txt&gt;</tt>".
This was capability needed by pikchr and backported here for safe keeping.
SQLite does not need this capability, though it doesn't hurt to have it in tree.

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