markdown-preprocess: add generic include mechanism

This is what was supposed to be an easy two-or-three-line
change to enable a more general-purpose include mechanism
than '@@option'; one that could include an arbitrary file.

This is commit 2 of 2, the "easy" part. Unfortunately, it's
not looking good. The source .md file has UTF8 checkmarks,
and nroff is not happy with those: the generated man pages
are gross.

Another problem: the source .md might need tweaking, because
we don't want a level 1 header in the man page. Obvious solution
is to make kubernetes_support.md a .md.in file as well, and
move the tables to a separate file (or files). Deferred for later.

Signed-off-by: Ed Santiago <santiago@redhat.com>

hack/markdown-preprocess

@@ -43,6 +43,10 @@ class Preprocessor():
                     _, optionname = line.strip().split(" ")
                     optionfile = os.path.join("options", optionname + '.md')
                     self.insert_file(fh_out, optionfile)
+                # '@@include relative-path/must-exist.md'
+                elif line.startswith('@@include '):
+                    _, path = line.strip().split(" ")
+                    self.insert_file(fh_out, path)
                 else:
                     fh_out.write(line)