Obey @exampleindent in plaintext and Info output

* tta/perl/Texinfo/Convert/Plaintext.pm (_convert): use exampleindent
customization info to increase the indent length for
%example_indented_commands.  Report from Rik.

* tta/perl/Makefile.tres, tta/perl/t/20preformatted.t
($nested_examples, exampleindent_set, exampleindent_zero)
(exampleindent_set_nestings, exampleindent_zero_nestings): new tests
of @exampleindent.