[master] build a text version of the guide

This was brought up at ISC All-Hands.

This will be included in the tarball.

For now this will be included in the repo too
(a different commit will add it).

I am using elinks for now. Note for now nothing in make forces it
to be rebuilt, so no tool is needed for this.

doc/guide/Makefile.am

 EXTRA_DIST = bind10-guide.css
-EXTRA_DIST += bind10-guide.xml bind10-guide.html
+EXTRA_DIST += bind10-guide.xml bind10-guide.html bind10-guide.txt
 EXTRA_DIST += bind10-messages.xml bind10-messages.html
 
+HTML2TXT ?= elinks -dump -no-numbering -no-references
+
+bind10-guide.txt: bind10-guide.html
+	$(HTML2TXT) $(srcdir)/bind10-guide.html > $@
+
 # This is not a "man" manual, but reuse this for now for docbook.
 if ENABLE_MAN