Allow specifying a base path for the generated pages

Allow specifying a base path for the generated pages, relative to the
output location of the original document.

If foo.html were to be output to out/foo.html and 'pages/' is specified
as pagesOutBasePath in foo.html, the generated output would be:
- out/foo.html
- out/pages/foo.1.html
- out/pages/foo.2.html
Allow naming pages with numbers only

This feature, combined with the one that allows specifying the base path
of the output pages could be useful to improve url readability in some
cases.

If foo.html were to be output to out/foo.html, 'pages/' is specified as
pagesOutBasePath and outputNumbersOnly is set to true in foo.html, the
generated output would be:
- out/foo.html
- out/pages/1.html
- out/pages/2.html

That, coupled with cleanurls allows getting urls like the following ones:
- <site url>/foo
- <site url>/pages/1
- <site url>/pages/2
Allow specifying a starting page number

If foo.html were to be output to out/foo.html, 'pages/' is specified as
pagesOutBasePath and outputNumbersOnly is set to true in foo.html, the
generated output would be:
- out/foo.html
- out/pages/1.html
- out/pages/2.html

And, if startingPageNumber were set to 2 in foo.html, the output would be:
- out/foo.html
- out/pages/2.html
- out/pages/3.html

The specified value for startingPageNumber must be a number greater than
or equal to 1, otherwise it defaults to 1.